A whole genre built around the witch/warrior, vampire and werewolf as a proxy for the hard-edged dame with a heart of gold (as the proxy for the (mostly female) reader), the wealthy, urbane rich guy and the tortured boy from the wrong side of the tracks. I was actually on a writer's panel with Kat Richardson about this particular trope at an SF convention in Seattle last year, and there's no question that it has taken over both the urban fantasy and romance genre like kudzu.
It's a formula, a certain degree of wish fulfillment, and yes, ultimately it is fantasy, but it also caricatures the caricature. It plays to the fantasy that the bad boys can be tamed by the right woman. Bad boys are fun - they are dangerous, adventurous, doing something that breaks out of the expectations of your family or culture. It's worth noting, however, that the stories become considerably less interesting once the bad boys have been tamed, which is why the series that feature this trope begin to get stale and tedious after the first couple of volumes, unless there is something else to carry it (for some reason, father/daughter issues tend to be the follow-through trope, for some bizarre reason).
= new reply since forum marked as read