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. As Leiter notes:
This guy sure is pissed, not without reason. The Bush shills and loyalists have coined a phrase as a defense mechanism against this kind of increasingly common reaction: they refer to "Bush haters," as though this kind of response is irrational and inexplicable. It doesn't occur to them that there are reasons people hate Bush, that people are responding to events, to facts, to stupid things the man and his Administration have done.
(I note that Leiter himself is commonly targeted by people committing the "fallacy" fallacy. People are put off by his abrasive tone and plain-spoken insults, and thereby conclude that he's committing "ad hominems", no matter the substance of his arguments. Of course, one may question whether it's the best way for him to persuade anyone who doesn't already agree with him -- but Leiter has explained that that is not his purpose anyway.)
Okay, so far we've distinguished "personal attacks" based on whether they form premises or conclusions of counterarguments. Only the former are fallacious -- and not even all of them are. I can think of two general exceptions. The first type is when the person being attacked will be responsible for carrying out the action being debated, and hence their character is relevant to assessing the likely consequences of the action -- I explain this in more detail over at Prior Knowledge, in response to a "fallacy" fallacy from Simon Clarke.
The second type is more complicated, and it was brought up in our political philosophy class last semester, where G.A. Cohen (I think it was him) uses it in relation to the incentives debate over tax. Anyway, the basic structure is this: a kidnapper has stolen your child; you will only get your child back if you pay the ransom, therefore you should pay the ransom. This seems like a fair enough argument, e.g. if made by the police who are helping you. But the normative force of the argument is completely different if it is instead made by the kidnapper himself. After all, he is responsible for making the unfortunate premises true. As such, his use of the argument is reprehensible blackmail, rather than helpful advice. That doesn't necessarily make it unsound. But it does mean that the person making the argument (i.e. the kidnapper) can be blamed for doing so. Cohen argues that when capitalists argue that they will work less hard if taxes are raised, this is a similarly reprehensible "threat" of an argument.
Anyway, the point to note here is that arguments are not simply the free-floating propositional structures that philosophers are trained to see them as. Sometimes it makes an important difference who is making the argument. As such, it is not always illegitimate to draw attention to this fact.