I look at it this way, you spend $2500 on a top of the line canon lens, a lens that is made with special glass and has thousands of hours of research and development into it, and now you've got a piece of shit glass thing on the front of it. Think like this, you're gonna go bang your old lady with a condom on, sure it'll work fine, but it's much better if you take that sucker off and hit it raw.
edit: it's one thing if you're using a polarizer or a red/yellow (for black&white) but uv's are pretty pointless these days, modern lenses are built much better and withstand alot more abuse. Hell, I've cleaned my L shit with a tshirt before, never hurt it.
Another thing that filters can do is cause reflections, here's a picture I took years ago, before I knew any better. I had a UV filter on the lens, and it caused the lights from the mojo 13 sign to reflect on the bottom of the pic. It took me a long time to figure out why they were coming out like that, ever since then I ditched my UV filters. Apparently I didn't know anything about white balance at the time either