I don't know if there's a name for this, but I definitely do it. I think it's perfectly legitimate in certain circumstances. For example, the more B is a subject of general dispute within the relevant grouping, and the more closely-linked belief in B is to belief in A, the more sound the heuristic. But it's not a short-cut to truth.

For example, suppose that you don't know anything about healing crystals, but are aware that their effectiveness is disputed. You might notice that many of the same people who (dis)believe in homeopathy also (dis)believe in healing crystals, that the beliefs are reasonably well-linked in terms of structure, and you might already know that homeopathy is bunk. Therefore it's legitimate to conclude that healing crystals are probably not a sound medical treatment - although you might revise this belief if you got more evidence. On the other hand, note that reversed stupidity is not truth - healing crystals being bunk doesn't indicate that conventional medicine works well.

The place where I find this heuristic most useful is politics, because the sides are well-defined - effectively, you have a binary choice between A and ~A, regardless of whether hypothetical alternative B would be better. If I stopped paying attention to current affairs, and just took the opposite position to Bob Crow on every matter of domestic political dispute, I don't think I'd go far wrong.