I'm still undecided on UV filters to protect the front element of my lenses. There seems to be 2 camps, for and against using protective filters.
On the plus side you have size & convenience (I find hoods both bulky: on the lens it makes you stand out like a sore thumb, in the bag it's a squeeze, in the pouch it doesn't fit - and finicky: annoying to screw on & off every time you take the camera out of the bag), protects against abrasions, interferes a lot less with a polariser than a hood, and as an extra bonus cuts through haze a bit.
However on the negative side is cost, extra glass to cause nasty refractions or worse (avoidable by making the cost problem worse ;-), vignetting, and as an extra bonus hoods protect against flare (even if it is minimal) - though of course there's nothing stopping you from having filter + hood.
I used to have a Hoya on my 70-300 and you really did notice the difference (in a bad way, especially loss of resolution). A guy from a shop (not where I bought it) said that was really strange and the filter might be faulty. I now have a B+W on my 24-105, I should do some pixel peeping on it someday I guess.
In the end I don't know if it is really an issue, I can't see Frank Cappa or Ansel Adams angsting over these kind of things (perhaps we angst over gear because we aren't them ;-). The arguments I've seen tend to border on the fanatical (as, I'm sorry to say, what I sometimes read about Nikon on these forums - a camera is just a tool, get over it ;-).
Thoughts on the matter...?