Ad hominem, but only after ad argumentum
Mark Shea links to a post by Zac Alstin on ad hominemheuristics:
The ad hominem heuristic takes into account a person’s views,
reputation, past conduct and circumstances as relevant information,
while still acknowledging that actual arguments must be addressed on
their own merits. The ad hominem heuristic is a basic premise
of political life: when workers’ unions support a piece of reform, it is
all but guaranteed that business groups will be immediately wary. For
most of us the quickest way to form an opinion on an issue is to see
where our enemies stand.
Which should give you a pretty good idea of the value of a quickly formed opinion.
While allowing that it's human nature to try to understand why other people do things we ourselves would not do, Zac Alstin rightly observes that
when we engage in these ad hominem heuristics it means we have –
rightly or wrongly – ceased to address or investigate the truth of the
issue itself. We’ve moved on from the actual argument to secondary
questions such as how much of a pillock you must be to hold opinions
contrary to our own.
That's the key point, I think. It can be useful to know why someone holds an opinion, but one of the uses of that knowledge is not to learn the truth regarding that opinion. To use Mark Shea's example, saying, “Joe’s wrong because he’s a stupid bigot,” can be legitimate if two conditions hold: first, that what we mean is, "Joe hold the wrong opinion he holds because he's a stupid bigot"; and second, that we already have a sound argument that Joe's opinion is wrong.