Some guys like such tanlines (ok, maybe not to such extent) so that they have a feeling that they see something that is hidden to everyone but them, that their girlfriends have something only to them. However, it's not the point in case of porn model ;)
Personally, I don't like them, they spoil the charm of such beauties as Nicole from 18magazine:
http://peachyforum.com/forums/thread/300194.aspxIt seems that she wears something like trousers on a beach ;)