Political correctness, doubt, and confidence

I've been following some of the conversation started by an article by Jonathan Chaidt opposing political correctness from a relatively liberal perspective.

So far, the best argument as to why not to use coercion to suppress speech still came from (relatively conservative Catholic) Russ Douthat though:

The strongest answer, as I’ve tried to suggest before in debates about pluralism, has to rest in doubt as well as confidence: In a sense of humility about your own certainties, a knowledge that what looks like absolute progressive truth in one era does not always turn out to look that way in hindsight, and a willingness to extend a presumption of decency and good faith even to people whose ideas you think history will judge harshly. If you just say, “I believe in free debate because I’m certain than in free debate the good and right and true will eventually triumph, and I know that coercion will ultimately backfire,” you aren’t really giving the practical case for coercion its due. Better to say: “I believe in free debate because I know that my ideas about the good and right and true might actually be wrong (or at least be only partial truths that miss some bigger picture), and sometimes even reactionaries are proven right, and we have to leave the door open to that possibility.”
The problem with political correctness, in this sense, isn’t that it necessarily hurts the causes it claims to advance; sometimes it doesn’t, sometimes it just helps them win. It’s that like most systems of speech policing (which of course held sway in traditional societies as well) it excludes the possibility that those causes might be getting big things wrong, and thus it hurts the larger cause of truth.