Amongst many expensive coatings applied to any quality lens will be one for UV anyway, so you don't 'need' one except for front element protection. Adding a UV filter will do no harm unless it's a real cheapo job. Good filters are made of glass of a similar quality to good optics, and like everything they don't come cheap.
Sticking a real cheapo bit of glass in front of an expensive lens will admittedly protect the lens, but it stops you getting the optimum image quality you paid all that dosh for too.
The other thing I'd say is you should always use a lens hood anyway, but it's even more vital with a filter fitted as you now have a much bigger area of glass right on the front of the lens just where it's going to get the most flare.