Being mean to someone who is not themselves being mean or manipulative is often not just counterproductive and self destructive (due to the reasons you mentioned), but also the result of personal weakness and lack of control. Meanness usually results from one of the following situations:

1. We feel angry and speak impulsively, entranced by our emotion.
2. We speak carelessly and don't realize the potential emotional consequences of what we are saying.
3. We are consciously and knowingly trying to hurt another person.

In case 1, our anger reflects a personal weakness in that our emotion prevents us from behaving level headily and making sure that what we say really promotes our interests. In case 2 we are speaking without awareness of the consequences of our actions, and hence again put our interests at risk. Only case 3 has a shot of being (at certain times) a good strategy, but in that case we must ask why we are consciously and knowingly choosing to hurt another human being, and whether such an action is ethical and justified.