My potters nips at everything, and by everything I mean rocks, powerheads, sps, glass, etc. Hes just looking for food, and doesnt bother anything really. I think people see this behavior, and are like OMG, NIPPER.
If hes not tearing off flesh, hes not bothering anything.
I think angels are like 80/20, and people just freak out over some of them that arent really causing any trouble, but there is always the chance that you get one thats bad.