The worst that can happen if a troll gets away with pretending to be, say, a reporter for some big newspaper doing a story on folk music, would be:
1) We'd all be nice to him and try to answer the question.
Some of us would be naive and some of us might just pretend to believe to keep the discussion from being sidetracked or looking stupid later on if the person was for real. There would be no way to tell which category the 'nice' people fit into.
2) We might sense a troll later on from the nature of their posts and bow out. It doesn't do any good to announce this decision. You'll either have people telling you you're paranoid or provoke a mob of Mudcat villagers with farm implements and torches heading off to storm the castle. The flaming starts, but it's not the troll who's responsible.
3) We might feel stupid for having fallen for it. Big deal. Being nice to someone by mistake is NOT something I'm ever going to regret.

On the other hand, if it's a real reporter and we go off on 'exposing' him, we do more harm than any troll could dream of.
See 3) above - we might feel both stupid and mean and wish we'd done 1).