The phrase [ad hominem] now chiefly describes an argument based on the failings of an adversary rather than on the merits of the case

All my arguments include logical reasoning in addition to the freely offered slander. And for some lucky few (such as xti), I have not offered any argument - only insult, so that is not really ad hominem either. ;-)

Now, if an insulting statement is true, and it can be demonstrated, then how does that harm my case? It makes me look mean, but that is already a known factor in my case.

Really, some people refuse to consider your arguments, and talking to them is absolutely pointless. Yet if you simply stop, that is seen as weakness.

If you continue to discuss in a reasonable tone while they prattle on, then you are clearly just wasting your time.

For a very few people, there is no good reason to address their points. Today there is only one person like that, though.