I chose fantasy, even though I don't play much with vampires at the moment and never with aliens (yet), but I like to have it in my game and mess around with it sometimes. Like @Dreamie209 said, I also like to play my game like a sitcom, and vampires are like a random surprise, for example, I just got a sim randomly pregnant by a vampire just for the laughter.
Another reason why I like the fantasy aspects in sims games is the inspiration it gives me for unique stories. Best example is TS3 Into the Future (you can call it "fantasy" I guess). It made me imagine and build up a whole science fiction story which I want to bring to paper eventually, without ever actually playing it out because at this state, it wouldn't even be possible with normal gameplay anymore. I'm a storytelling player, and I need to imagine stories to stay interested in the game I guess. Sure, you can make up stories with realism as well, but fantasy and non-realistic stuff can just inspire me more. Maybe that's the reason I'm more interested in a pack with witches or time travel rather than laundry or seasons. It just has more storytelling potential. I'm also really interested in the new world adventures styled gp that's coming out soon :)
I think I just went a bit off-topic, so to summarize it: I am a realism player that needs fantasy to spice things up. :)