In a community of people who could potentially work together to reason about possible world futures, being disrespectful - which is a more endemic sin here than being "mean" - causes people to change their LW goal set from, say, "save the universe from rogue AIs", to "save the universe from rogue AIs, and discredit person X at any opportunity." And it's not even being trite or irrational. If you know that someone has a low opinion of you, and shares it freely with others; then advances in that person's career may hinder your advance in your own. In a small world of people doomed to encounter each other over and over at different conferences and events, displaying open contempt is stupid. It hurts the entire community.