My sense is that critical thinking and informal logic classes stress the evaluation of arguments, not arguers. This is fine as a starting point, but as a long run strategy, it ignores the fact that we have very often to evaluate arguers. Someone who makes good ones, like someone who can throw good pitchers, is a good arguer; someone who makes bad ones, is a bad arguer. It's a kind of skill. The judgement about the person arguing strikes some, however, as having too much of an ad hominem character. But ad hominems are not by their very nature fallacious. They're fallacious only when the ad hominem judgement has no relevance to the truth or falsity or reliability or whatever of what a person is saying.
In light of this, consider George Will's latest attack on his favorite hollow man, "progressivism."
In 2011, for the first time in 62 years, America was a net exporter of petroleum products. For the indefinite future, a specter is haunting progressivism, the specter of abundance. Because progressivism exists to justify a few people bossing around most people and because progressives believe that only government’s energy should flow unimpeded, they crave energy scarcities as an excuse for rationing — by them — that produces ever-more-minute government supervision of Americans’ behavior.
and then later:
An all-purpose rationale for rationing in its many permutations has been the progressives’ preferred apocalypse, the fear of climate change. But environmentalism as the thin end of an enormous wedge of regulation and redistribution is a spent force. How many Americans noticed that the latest United Nations climate change confabulation occurred in December in Durban, South Africa?
Let's put this another way. A person who makes up phony opponents (hollow men) merely in order to knock down their imaginary arguments with demonstrable scientific falsehoods is a very sorry arguer. That's an ad hominem.