This LJ entry makes a number of highly valid points, and the last paragraph more or less sums up my own thoughts on the matter. (For people too lazy to look, he says: "Personally, I find burning the flag to be stupid, childish, and unnecessarily offensive to many good people. But I still believe that we should be able to burn it whenever the hell we want. If we can't, then it doesn't stand for much at all. And the legal right to burn the flag is one of the best reasons not to burn the flag.")