One issue I left dangling (relegated to a footnote here actually) is whether to understand the SMT as a methodological or a metaphysical thesis. The difference lies in evaluating the SMT wrt its fecundity or wrt its truth. I am partial to the first reading. In fact, I find it hard to see how the metaphysical version of the SMT could be true. Let me elaborate.
Minimalists like to say that that Minimalism is a program, not a theory. I actually have some reservations about this claim for programs are vindicated to the degree that they generate interesting and true theories, if not right away, then over a reasonable time span. The Minimalist Program (MP) is now approximately 20 years old, so we should by now be evaluating it in terms of its theories. As I believe that there are a lot of pretty good and empirically interesting (even true) proposals that the program has generated, I think that the “program” line can sometimes be a dodge. However, whether this is true or not, there is something important about the ‘program’ vs ‘theory’ distinction that is relevant to what I want to say here. The main distinction between programs and theories is that theories are things that can be true or false whereas programs are things that are fertile or sterile. As such, programs generate research methodologies, ways of approaching questions that can lead to theories that are truth evaluable. And one important feature of a good program is that the methodologies do actually function as productive hypothesis generators. One of the features of minimalism from the get-go has been its ability to suggest interesting avenues of linguistic investigation. It has done so in several ways.
First, as Chomsky has liked to stress, it keeps us honest. Often our explanations are the same order of complexity as the phenomena they aim to explain. This may or may not be useful (re-describing something can be a crucial step to explaining it), however, it is unlikely to be explanatory. It’s never a good idea to explain N data points with a theory that has N degrees of freedom. At any rate, early minimalism stressed the methodological virtues of simplicity and elegance and tried to motivate how they might be operationalized in the context of late GB syntax. Chomsky’s 1993 paper, the one that launched the enterprise, was a very good guide of how to do this, deploying Ockham’s razor to great effect in cutting away some GB underbrush. Chomsky’s basic point was methodological viz. if we want our explanations to explain then they cannot be as convoluted as the data they care about. And he observed that firmly keeping this in focus can lead to a significant explanatory boost, i.e. less can be more, a whole lot more.
Second, MP started people (like me) thinking about the virtues of reduction/unification. Though never absent even in earlier work, this kind of project makes deep sense in the minimalist context. Why? Precisely because it urges that we go “beyond explanatory adequacy,” to use Chomsky’s terms. GB was mainly concerned with Plato’s problem. This problem is “solved” once something is put into UG for by assumption things in UG need not be learned. This often has the effect (I am confessing here) of removing the incentive for developing swelt, streamlined, elegant accounts. So long as the principles can be shoved into UG their ungainliness fails to generate much empirical friction. To put it crudely (something many of you might think I do all too well) GB theories were to elegance what the lunar module was to aerodynamics (Did you ever see the damn thing? It looks like a pile of mechanical garbage wrapped in tin foil, see here) and roughly for the same reason. The module didn’t need to be nicely shaped to move efficiently through space as way out there space is a frictionless medium. In a sense, that was also true of GB; once inside UG, the shape of the principles didn’t much matter for Plato’s Problem.
Now I don’t want to overstate this. Linguists have always cared about elegance, simplicity, redundancy, etc. However, MP greatly raised the status of these virtues. These virtues fueled the impulse to unify the principles of UG and unification became empirically important when one worried not only about learnability issues but also about how FL/UG itself might have arisen. I’ve talked about this (no doubt too much, but as you can see I am obsessed by this) elsewhere (here) so I will drop the issue now. But I bring it up because it bears on the correct interpretation of the SMT.
One way of thinking about the SMT is along the lines of these more general desiderata. In other words, the SMT is an injunction to look for examples where interface properties reveal representational structure. The PLHH work shows that the ANS+visual system can tell us quite a bit about the nature of semantic representations (aka linguistic meaning) and work on parsing and acquisition can do so as well wrt syntactic representations. When such things are found, they can be revealing and the SMT, viewed as a methodological precept to look for such, can be, and has been, quite fecund, especially in forcing different kinds of linguists (syntacticians, phonologists, psycho types, and even neuro types) to ask how their projects and assumptions fit together. In short, as a guiding methodological principle, the SMT is a winner: fecund? Check ✔.
What about a metaphysical thesis? Here, things get a whole lot murkier. Recall that the SMT is supposed to be the thesis that the grammar is the optimal solution to interface conditions. One way of reading this is that the interfaces cause linguistic representations to have the properties they do. But what would it mean for this to be true? I really don’t know.
There is one possibility, the standard Darwinian one in which over long periods of time the interfaces chisel away at the rough edges of FL/UG (and vice versa) till they fit snugly together (interface requirements accommodating themselves to features of FL/UG and properties of FL/UG accommodating themselves to features of the interfaces). Maybe, but recall a good deal of Darwin’s Problem in MP rests on the premise that FL/UG popped up pretty quickly and so there was no time for the Darwin’s selectionist mutual accommodation to effectively operate. Without this Darwininan solvent, any fit that exists between interfaces and FL/UG will be quite adventitious. In fact, I would expect such perfect fit to be the exception rather than the rule and I expect that there will be/are many many interfaces with which the resources of FL don’t integrate at all well with systems that (try to) use them. I can personally attest to the fact that my “dance module” is almost completely inured to verbal instruction. So, as a metaphysical thesis, I see no reason to believe that the SMT is even roughly correct. Or if it is correct it is total mystery why it is or even could be. It would be too damn amazing were FL/UG to be just what every interface ordered. This would be super-intelligent design! This is why I find the SMT to be a pretty poor metaphysical thesis: from where I sit, it has all the hallmarks of being obviously false (indeed, incredible).
Is there anything paradoxical about a principle being methodologically fecund though metaphysically false? Nope. Fecundity and truth are related but distinct evaluative dimensions. To repeat: programs/methodologies fecund, theories/proposals true/false. So qua methodological precept (viz. look for this!) the SMT is a powerful injunction, but qua metaphysical thesis, not so much.
Let me put this another way by considering an analogy between the SMT and the Anthropic Principle (AP) (here). The AP can be used to deduce the values of attested physical constants. How? Well, the values must lie within a certain range in order for (conscious) life to be possible. As the universe clearly contains (conscious) life (i.e. us, well on some days at least) this fact can be used to specify a narrow range of values for the attested physical constants (e.g. the fine structure constant). As a methodological principle, AP seems unexceptionable. Given that we are here, of course the universe must be hospitable to us and this means that the physical constants must have hospitable-for-us values. However, as a metaphysical principle AP has a decidedly mystical air (e.g. the universe is “compelled, in some sense, for conscious life to emerge” (Wikipedia). Note the “in some sense,” always a sign that things are getting weird) that has a distinct theistic odor suggesting intelligent design. The SMT is similar. If FL’s products fit an interface transparently there is a lot to learn about the fine structure of the representation. However, this is not because the interface causes linguistic representations to have the features they do but because in the domains where the SMT holds features of the interface and features of the representations are very closely correlated. Thus, knowing the properties of one can tell you a lot about the properties of the other. In other words, where the SMT holds features of the interface can be used to probe features of the linguistic representation. And just as our existence has implications for the values of the physical constants (at least in our universe) per AP, so too do properties of SMT compliant interfaces have implications for the properties of linguistic representations, even if metaphysically speaking both the AP and the SMT are false. 
In sum, even if the general metaphysical version of the SMT is false, there is reason to hope that some interfaces will fit with FL/UG tightly. The properties of these can then be used to plumb the internal details of FL/UG (and, of course, vice versa). These domains of investigation will then be closely integrated, allowing for the development of richer theories of both FL/UG and the relevant interface.
Methodologically, one can go a little further and elevate the SMT to a methodological ideal. In particular, we can take as a default assumption that, for any given interface, the SMT (viz. the Transparency Thesis) holds. It should be easyish to disconfirm this if false (and I suspect that it will be often false), so it is a good 0-th level assumption to make. In the meantime, whether the SMT holds or not for a particular interface, we will find something interesting, and that’s what makes it an ideal methodological principle.
No doubt, there are other interpretations of the SMT that are more metaphysically charged (see Introduction of this for example). There are times when Chomsky’s allusions to third factors and snowflakes can carry this kind of tinge (there are also times when he resiles from this interpretation and explicitly adopts a methodological stance wrt MP and its precepts). For me, it is comforting to be able to interpret the various programmatic precepts in methodological terms. Why? I understand these and can see how to use them to generate research hypotheses. Seen from this perspective, the SMT is a very good way of framing linguistic questions, even if it is metaphysically very far fetched.
 This post developed from conversations that I had with Paul (the ‘P’ in PLHH) about the Interface Transparency Thesis and the SMT. It goes without saying that he is completely responsible for any dumb ass thing that I say here. Don’t like it, complain to him.
 A possible counter is that it’s too early to engage minimalist themes. Perhaps. But if so, then it’s not really a program either, more like a vision or dream.
 Now for a mea culpa (footnotes are good for this): (here) I said that the features of the ANS+visual system explain the features of L. This strongly suggests that they are the cause of those features in L. If the above is right, this is very misleading and I accept full responsibility for misleading you. I am so contrite that I am sure you will all forgive me. Thx for your indulgence. What we can say is that given the ITT we can deduce some features of L by noting features of ANS+visual, but in this case deducing X does not amount to explaining L (think heights of flagpoles and the shadows they cast).