Further, some fallacies, like ad hominem are poorly understood, so when an opponent says you're wrong because of this this and this therefor you're an idiot, the poor victim of the ad hominem feels like they can claim victory over the argument. When in reality ad hominem refers to the dismissal of an argument by just insulting the person. Time and time again you see someone exasperated by the crank who won't turn despite being shown again and again where their error is, and finally just call the guy an idiot. That's actually not an ad hominem. That might be totally true and highly relevant to the argument at hand. Sometimes people are just too stupid or too ignorant to realize when they've been soundly thrashed, and true cranks will stubbornly go on, and on and on...
While it's a common misconception that ad hominem involves insult, this is not actually correct. You can commit ad hominem by complimenting a person as well, for instance,
Tom's argument for his explanation of Sam's behavior is faulty because Tom, being nice, can't understand how a wicked person like Sam thinks
will, in any circumstance where Tom's niceness is not actually relevant to evaluating Tom's explanation, be fallacious. This type of ad hominem is arguably less common, since in most cases we don't usually make the objection that positive characteristics of the arguer vitiate the argument, but it is certainly ad hominem, because it evaluates the argument negatively in terms of the arguer's character.
But MarkH is quite right both that this is not necessarily vicious and that ad hominem is poorly understood. I roughed out an account of ad hominem fallacy a while back that makes an attempt to deal with some of the problems that an account of ad hominem fallacies must face.