For the first three seasons this show was a hardcore urban fantasy with gore, guns, horror, extreme darkness, hot woman, sweet cars, a soundtrack that got me into classic metal (Dio is god the rest of sabbath, Judas priest and ac/dc his loyal subjects) and some moments of serious depth (children shouldn't play with dead things anyone?). As of season 5 it is-an albeit believable- wangstfest with the exception of Castiel. Why?
The shoehorning in of characters like Bela, who seem to only exist to be young and attractive and female, when the "girl of the week" format did it so much better than a sudden new lead/recurring character with very little to do in most episodes (and the woman in question was usually more likable, too). Especially when it comes at the expense of secondary female characters like Ellen, who was interesting and attractive (like women over thirty-five regularly are, despite the writers' apparent opinions to the contrary) and actually had a reason to show up, if anything, more often than she did. I'm not saying there's no place for female leads in the show, just that being a good-looking young woman isn't by itself enough to make a character interesting over a prolonged arc.