The problem is that people commonly object to any form of insult as ad hominems. But this is mistaken. An ad hominem fallacy is when you reject your opponent’s argument because of some characteristic of the advocate that is irrelevant to the content of the argument made. In general, what matters is the argument, not who makes it. (I will mention some exceptions below.) But not all “personal attacks” take this fallacious form. Rather than saying “you suck, therefore your argument does”, one might instead provide an adequate counterargument, then append: “your argument sucks, therefore you do”. Such gratuitous insults may be unwise, but the counterargument doesn’t depend upon them, so it’s a mistake to object to the counterargument (and ignore its substance) on that basis.
Further, I think insults aren’t always inappropriate. Creationists and homophobes, for example, tend to make revealingly bad arguments. In critiquing these arguments, we first aim to show why the conclusion doesn’t follow. But we are also in a position to draw a conclusion about the character of the person advancing the argument. Many arguments are so bad that they could not be honestly made by any informed rational person. Thus anyone who makes them must be either stupid, ignorant, or dishonest. In the course of criticising such an argument, a partisan may wish to point this out, just to emphasize how incredibly bad the argument really is. I don’t think it’s necessary wrong to do so. Some positions are so lacking in rational warrant that they deserve our scorn.
For an example of the “fallacy” fallacy in action, just look at political blogs. Liberals tend to insult Bush a lot in the course of their arguments. Conservatives then respond by dismissing them as “Bush-haters”. But in fact it isn’t the “Bush-haters” that were committing ad hominems – after all, they had arguments to back up their insults. On the contrary: the conservatives are effectively arguing, “You hate Bush, therefore I don’t need to address your substantive argument.” – and that is a clear case of argumentum ad hominem.
Unfortunately, pointing out this “fallacy” fallacy to a person capable of making it will probably result in further hostility or at least confused looks (sort of like that moment when Jon Stewart suggested to Richard Branson that he use his billions to fund the development of alternative energy sources—it did not compute). I think the sooner us rational types get over the idea that everyone should be (or can be?) rational the better chance we will have of finding a way to shift out of this paralyzing dynamic.