I believe "nice," makes an excellent default, and I think these arguments are good ones, well-presented. Not-niceness is sometimes an effort to signal intelligence, I think; it's not particularly effective at that.

It's important, too, though, to recognize when niceness doesn't pay the utilitarian bill:

1. Trolls. In any environment, people interested in reactions occasionally wander in. These people should be banned and ignored. Shunning is not a particularly nice thing, but even polite feedback is feedback. Do not feed these.

2. The ineducable. Suppose a person asserts that the Monty Hall problem results in a 50-50 chance of switching or not switching. One or two efforts to educate nicely is good. Additional efforts are wasted and unproductive.

3. Evil. Deliberately dishonest people are far rarer than alleged on internet fora generally, but it happens. Shaming people who are genuinely bad actors is fine with me, thanks.

4. I've probably missed several. Assistance welcomed.

Overall, I'd say LW is a particularly civil corner of the internet, and I've spent time at some uncivil places. The other side of pro-niceness posting is that assuming unkindness or bad motives is not a good default for the reader; assume the other person's directness isn't not-nice.

I'd also say that I think niceness is more important for persuasion on the net than it is in person; people who know you personally might assume more internal niceness even you're sometimes a condescending, sarcastic, know-it-all. (Really.) That sort of instinctive goodwill towards one's fellow man is tougher to generate without a handshake.