When a high page rank site links to another site it affects their rating; I don't want to allow this for tiny little crap sites which will be gone in two weeks; I like to encourage sites to sign an affiliation agreement before blindly sending them traffic (and thus vouching that I want to refer them)...
You'll actually find several sites online who will pay you for linking to them if your site has a PR of more than a 3... It's crazy how many people just go and spam sites which just allow any joe schmo to post links (and not nofollow)
