Sharing your knowledge is fun. It's a long-time net tradition. And it makes the world a better place. Tact is not its objective. Not at all.
Flames can be lots of fun, both to write and to read.
And the recipients of flames sometimes deserve the heat. It's unfair to the other members of the group. And while flame wars can initially be amusing, they get boring very quickly to people who aren't involved in them. They're an unfair monopolization of bandwidth.