By calling someone a name, what some people attempt to do is encase their opponent under a label deemed by society to be negative, minimizing them and whatever they were standing for. In reality, by doing so, they are allowing themselves to being labeled, and also lose both the high ground and the argument. Calling someone a name equals an immediate forfeit and the validation and lionization of the other side.
Worst of all, however, should be the following: calling someone names is a sign that they are neither smart nor witty enough to defend themselves with a logical argument.