Ad hominem arguments are chiefly problematic in pure logical discourse. In normal human rhetoric, the reputation of the person making the argument often factors in to whether listeners view the argument as truthful. It's not a problem to point out things that reduce the impact of reputation on rhetoric.

Essentially, any time a public figure says something, there's an implicit, "I have been truthful before, I am being truthful now," leading the statement. If someone's opinions are self-inconsistent, it's important to know that one can discount that implied statement.