Can't this be said of just about all straw men ? Yes, setting up a straw man may be instrumentally rational, but is it the kind of thing we should be applauding ?

Agreed. I don't necessarily approve of this sort of rhetoric, but I think it's worth trying to figure out what causes it, and recognize any good reasons that might be involved. (I also don't mean to say that people who use this rhetoric are calculating instrumental rationalists — mostly, I think they, as I alluded to, don't recognize the possibility of saying things representative of and useful to an outgroup without being allied with it.)