For instance, there are a wide variety of rhetorical strategies that manifest responses to the realization that oneself and one’s opponent are talking past one another, that is, are conceiving the terms of the debate according to incommensurable schemes. Seen in this light, Motte-bailey can be a useful and necessary means for finding a bridge, a code of translation , between the two worlds. — Joshs

I was thinking of something similar. I agree that the motte-and-bailey model is useful to overcome incommensurable schemes, but the motte-and-bailey approach may also be an integral ingredient in the natural ebb and flow of philosophical dialogue. If this is correct then it will be difficult to distinguish the fallacy from the natural ebb and flow of philosophical dialogue.

For example, upon reading Plato's dialogues we might reasonably conclude that the motte-and-bailey fallacy occurs on every page, but it is worth noting that this is also just part and parcel of philosophical dialogue. If such dialogue can be compared to martial arts, then there is a way in which each party is constantly alternating between attack and retreat. Part of this involves testing one's opponent by venturing into more controversial territory and then retreating back to a more easily defensible position. It is precisely this ebb and flow which produces philosophical knowledge and understanding.

That's not to say that there is no such informal fallacy as the motte-and-bailey, but it would be easily confused with acceptable ebb and flow. For this reason it is the sort of fallacy that would be useless in the midst of an argument. In the heat of argument the accuser will be likely to misuse the fallacy and the defendant will be unlikely to accept the charge. Yet when analyzing an argument from a third-party point of view the fallacy could be useful.

The core question would be whether the motte and the bailey are equivocal or "analogical" (i.e. interrelated and defensible). Since that question is already an essential hinge of the main argument, leveling the motte-and-bailey accusation in the midst of an argument would seem to be a form of begging the question. The fact that there is widespread disagreement about whether the example in the OP exhibits fallacious reasoning is a case in point. Generally fallacies need to be easily identifiable, and because of this I am wary of fallacies that are too subtle.