It would, for instance, suggest that you can't have a "fruitful discussion" about installing wheelchair ramps with people who identify as disabled

I would say that self-identifying as disabled is not quite the same thing as self-identifying with a group which believes in / advocates for rights for disabled people. Getting back to the religion analogy, one can ask about having a discussion of religion with a Jew -- in such a situation it would help to know if the individual is Jewish like Albert Einstein or Jewish like Moshe Feiglin.

you can't have a fruitful discussion about a topic when any one party to the discussion presumes that another party's purpose in entering the discussion is illegitimate.

Perhaps, but what if that's just a side effect of the heat generated by a controversial issue?