I used to be of the impression that it was never okay to be impolite. I based this opinion upon the present state of political discourse and my own personal belief in “frenemies” as an ideal. “Surely,” I had said at the time, “political discussions would be a lot more productive if everyone involved would just cool their jets and engage with each other in a respectful, and yes, polite manner.”
And in fact I stand by that to a large degree. I don’t honestly believe that shouting your point in an insulting or demeaning manner makes your opponent more likely to come over to your way of thinking. Quite the opposite, I think it makes them more likely to dig-in their heels against a percieved enemy.
However, there are occasions upon which being polite or civil would only be a waste of time. As a rule of thumb, when I have decided that my opponent is not arguing in good faith, that is the time when I will unleash the dogs of mockery.
The problem, however, is that this distinction is not always clear. When someone keeps arguing a point that has been refuted a thousand times, it’s possible that they’re arguing in bad faith; on the other hand, it’s also possible that you are not explaining you refutations clearly enough, or indeed, that your opponent is earnest but just plain stupid. You must make own judgements in these cases.
On the other hand, sometimes it’s totally obvious. Like, for example, when you’re reading the comments following an article about tax policy and the same “genius” has posted approximately thirty billion comments about the fact that Obama once ate a dog. In that case, the only reasonable response is to make a number of increasingly crude jokes about his mother.