Those are good questions. The point about the group dynamic in particular is one that came up in the comments elsewhere ... the fact that regardless of the person's intent, if they're derailing/destroying the discussion or driving away others, then that's something that needs to be addressed.

I know I have a hard time with the Men's Rights people because usually they're people who don't actually read or follow my blog; they just heard about something I posted, often days or weeks after the fact, and just HAD to rush over to tell me how wrong I am. I'm having trouble finding words to explain why I have less patience for that than for someone who's already a part of the conversation and disagrees with me...