I think there are interesting arguments to be had about meritocracy for example. But in my experience talking with critical theory folks, they have a different criterion for evaluating arguments: something like "has this argument historically served to reinforce oppression or to challenge it", instead of "is this argument true". So inviting them to a meritocracy debate is like inviting the guy who plays a saxophone solo whenever he hears any six-letter word - now everyone has to watch what they say, for reasons unrelated to truth.