Ad hominem arguments can be logical, but only after we have addressed the issue at hand.

Playing the man, not the ball

When we engage in these ad hominem heuristics it means we have – rightly or wrongly – ceased to address or investigate the truth of the issue itself. We’ve moved on from the actual argument to secondary questions such as how much of a pillock you must be to hold opinions contrary to our own.

http://www.mercatornet.com/articles/view/playing_the_man_not_the_ball