I found this illuminating, and agree that the use of the motte-and-bailey concept you describe isn't fair or helpful. But I'm not convinced the real problem here is with the appeal to the motte-and-bailey fallacy.
I find the motte-and-bailey concept quite helpful, but you can only legitimately pick out a motte-and-bailey if the self-same person is guilty of shifting between the two within the same work (or within multiple works intended to be consistent with one another). It's extremely common for individual people to personally motte-and-bailey within the space of a conversation (or 5-page paper), and having a concept to describe that is quite helpful.
So it seems to me that the real problem is not with the concept of motte-and-bailey arguments at all. All the actual problems are located around the tendency to hold every individual who calls themselves an X accountable for (simultaneously) all the opinions ever propounded under the label of X. The tendency to charge adherents of X with a motte-and-bailey fallacy even if they've not personally committed one yet is just one of many instances of this general tendency. The concept of a motte-and-bailey fallacy is no more at fault than the concept of an inconsistency is.