I mean in private conversation, not in public discourse, and this is not to their faces but rather behind their back. And with at least a modest amount of meanness, I am not talking about criticizing their ideas. Here are some reasons not to criticize other people:
1. “Complain less” is one of the very best pieces of wisdom. That is positively correlated with criticizing other people less, though it is not identical either.
2. If you criticize X to Y, Y wonders whether you criticize him to others as well. This problem can increase to the extent your criticism is biting and on the mark.
3. Criticizing others is a form of “devalue and dismiss,” and that tends to make the criticizing people stupider. If I consider the columnists who pour a lot of energy into criticizing others, even if they are sometimes correct, it isn’t so pretty a picture where they end up.
5. Under some moral theories, X is harming Y if X criticizes Y, Y doesn’t find out, and Y faces no practical penalties from that criticism (for an analogy, maybe a wife is harming her husband if she has a secret affair and he never finds out about it).