I touched upon this briefly in my post about "bipartisanship", but never felt I could get what I was trying to say about civility across clearly enough to justify a post of its own.
Yet today, I find Coturnix commenting here, and he expressed it almost perfectly. So I'll just quote him:
The call for civility is the method for the privileged to exclude the outsiders from conversation, because the only way to shock them out of their privilege is to tell them to fuck off.
Dammit, I wish I had said that.