Ad hominem isn't always a fallacy. Indeed: it's only a fallacy if you try to use it as proof of a claim. As a probability indicator, it's fantastically useful

Ad hominem isn't always a fallacy. Indeed: it's only a fallacy if you try to use it as proof of a claim. As a probability indicator, it's fantastically useful: if person A is right 80% of the time when they make a claim, and person B is right only 20% of the time (assuming all else is equal), then you'd be a damn fool not to incorporate that information into your estimate as to whether something is true: person A is four times as likely to be right, so in a disagreement between the two you should always side with A. I'm getting a little tired of people shouting "ad hominem" every time the credibility of the author is challenged. The fact that someone doesn't believe in evolution absolutely should make me take their thoughts on global warming less seriously; similarly, if someone has displayed an extremely immature, counterproductive, or selfish approach to relationships, you should probably take any relationship advice they give with a serious grain of salt.