I'm going to say your interest in anime and werewolves and vampires are connected.
I hate anime so that might play a roll in it.
Either way, it would be interesting to see a werewolf or a vampire in Skyrim, I will admit. but I do not feel it be necessary.
As someone who hates anime and is very much interested in the occult (particularly the real history behind it, rather than movies set in modern times) I'd have to say that's a fairly tenuous link. While there are animes that deal with vampires and werewolves, it's probaly about the same percent as western media. Giant robots, and schoolgirls are much more strongly representative of that particular genre.
And as far as "not necessary" the game would be fairly boring without any of that kind of feature. So while melee combat > vampires or magic use > vampires, I could not say trolls > vampires, or even dragons > vampires, although I'm sure many people would (not nearly all of them, though).
They are good as high level monsters, being more powerful than natural foes, and as I've said before are well represented across history and cultures. Also, because both monsters are human in nature, it adds that special twist. And being able to play as one is actually one thing that can be used to define TES from other RPGS...there are other games that let you play a vampire, but none that let you choose if that is a vampire archer, swordsman, mage, live in an open world, and eventually get cured.