An informal poll over on my Facebook led to people expressing a marked disinterest in reading a fantasy novel with modern swears (though many so-called modern swears date back hundreds of years). I myself have occasionally found it off-putting - at least until I wrapped my brain around the idea that the novel in my hands was going to contain such words, and moved onward to enjoy it.
But if the purpose of including profanity, vulgarity and obscenity in a fantasy novel is to provoke an emotional, visceral response in the reader, wouldn't it be advantageous to actually write the characters' dialogue in the sort of language that would do so? In other words, the difference between "that fornicating buggerer" and "that fucking faggot" is immediately apparent. The former is an intellectual exercise in the use of my Thesaurus; the latter, well, you be the judge.
And then there's the question of inventing all-new swears, like The Wheel of Time's "blood and bloody ashes", or at least replacement words such as Battlestar Galactica's "frak" or Firefly's "gorram". How intrusive are they? Do they immerse the reader in the world, or shock them back to reality?
My gut is saying to use modern swears. My head is saying not to.