Speaking as somebody who frequently engages in non-nice methodologies:

Niceness is more convincing. Way more convincing. And if you can get somebody to be mean enough to you, while you're being nice, that somebody feels like they should defend you, cognitive dissonance will push them to believe in your beliefs a little bit more.

If some people are nice, and some people are mean, we're injecting some very subtle irrationality into people reading our discourse.

So there is an advantage in picking one and sticking to it. (Or my policy, which is to match the tone of my opponent as well as I can.) And niceness is probably an easier schelling point that meanness.