Well, a spammer is always a bad neighbor, because he will get the IP banned. I would imagine that when buying "off shore" hosting results in bad hosting neighbors too, in the sense that their sites are involved in questionable activities, which always means a risk of some public authority seizing the server.
Some people are worried about not sharing the server with adult-type sites. I doubt there's much risk of any ill effects from that, but I can definitely understand a church wanting to make sure that their website's data is not millimeters away from some video of a moaning "lady".
