I'm not sure if this was a rhetorical question or not... so I'll reply.
At one of my previous jobs at a web hosting company, we had the privledge of sitting in on a discourse about Google
SEO put on by one of the guys at google. (no not the main guys, I don't remember his name, but he was very informative). He answered a myriad of questions posed by us in the room and explained a lot about how PR works and really focused on link backs.
He made it abundantly clear that a link back is a link back, regardless of where its from. It can't hurt you as you often can't choose who links to you. He stated, just view it as a compliment. Besides, you really can't prevent people from linking to you.
It certainly is true that reputable sites will help you with ranking and traffic, but **** sites, smut sites, or spam sites dont' hurt your ranking. But I would strongly advise not linking back :)
Also quick note that was also mentioned... link backs, at least by google, are each counted as a vote. (numerous links on the same site though are usually just 1 vote), each vote you have gets you closer to a higher PR. Many people aren't that concerned with PR but I thought that was interesting.