Ad hominems get a bad rap.
Specifically, there are instances where knowing that the person who thought up an idea has certain flaws is very useful in evaluating the idea.
In the best case scenario, I can evaluate every argument I hear on its own merits. Unfortunately, I'm often too busy to put enough time into every argument that I hear. I might just read enough of an argument to get the gist, and then move on to the next thing I'm interested in. This has bitten me a few times.
If I know that the author of an article is intellectually sloppy, that actually helps me quite a bit when it comes to evaluating their arguments. I'll put more time into an article they've written, because I now feel that its more important to evaluate it for myself.
If I know more specifically that an author doesn't understand supply and demand (or whatever), then that tells me exactly what parts of their argument to hone in on for more verification.
The general case of just dismissing an argument because the person making it has some flaw does still seem bad to me. It makes sense to know what kind of person is giving the argument, because that can point you at places that the argument may be weakest. This allows you to verify more quickly whether you think the argument itself is right.
Ad hominems shouldn't end an argument, but they can be a useful argument direction-finder.