People get turned on by different things. What they get turned on by has a habit of creeping into their work. Take Quentin Tarantino's Death Proof. Did you notice all those long lingering shots of beautiful women's feet hanging out of car windows? And this scene from From Dusk till Dawn gains a whole extra context to my formerly innocent little mind. Who says writers don't have any perks.
A little fetish fuel can add some spice to a story. Where it doesn't work so well, in my humble opinion, is when it becomes the sole focus of the story. In those stories I find I can't relate to the protagonist at all because I don't get the fetish and therefore the character's actions come across as odd. Fine for people in the niche, but of little use outside of it.
Playing through Monster Girl Quest reminded me how smart a lot of Japanese creators are at playing to the fetishes. Loli's, Big Tits, Bondage, Giantesses, Mind Control - take your pick, there's something for everyone. Darkstalkers, a considerably more mainstream game, gave us the iconic succubus, Morrigan, but there's a whole host of other quirks to add a bit of extra spice. Most of the comedy anime/mangas featuring harems will pull the same tricks to ensure everyone has a character to latch on to.
That isn't to say these things are innately perverted (okay Monster Girl Quest is - it's designed to be). From Dusk till Dawn is still a tense crime movie that morphs into an outrageous vampire film halfway through. With a bit extra if you happen to like feet. It's spice. If it's so much the audience starts to feel a little icky, then you've dumped too much curry powder in the Balti.
Unless you want...ahem...to make people feel a little icky.
I like mixing up horror and erotica for the mindfuck factor. Targeting fetishes plays an important role in this. What's a turn on for one person, can be twisted into something creepy and horrific for somebody else. One's reading it for the sex, the other for the horror. They're both getting something from the same story. Everyone's happy (at least until Horrorhead rears up and squicks everyone out).
My own weakness. With story titles like Crushed Between Her Breasts and The Biggest Tits in the World, it's all rather boring and obvious really...