I don't want to derail this thread with filter talk...but I've tested a few filters back-to-back and some actually ADDED contrast/sharpness to my lenses...don't know how...but they did. They're not all evil. If you want a UV filter, get a high quality one and you won't be disappointed.
(With that said, I use UV filters less and less...only have it on my 16-35L now to complete the weather sealing/I use that lens the most, and judging by how quickly I destroyed the filter - tons of hairline scratches, from sand, I'm assuming! - , I'm glad that's what's taking the beating, not the front element).