There is no doubt that UV/Skylight/Protective filters can degrade the image. I use them anyway. Always have. Always will. Here is why.

Nikkor 24/2.8, splashed with champagne spray in a locker-room following a baseball team victory. I had to turn the focus ring about 200 times to loosen it up but no fine wine made it inside the front lens element.

Nikkor 80-200/4, left on the bar of a local establishment and inadvertently knocked off on a tile floor. Bent the lenshood and smashed the UV filter. Not a scratch on the front element.

Nikkor 180/2.8, put on the roof of my vehicle during a lens change and falling to the pavement on the front element, then rolling under the car. Broken glass everywhere. It was the filter only. Front element unharmed.

Same lens on a Nikon F2 being hit at a football game. I fell onto the lens, the front of the camera was broken completely off, still attached to the lens and the lens was buried in the soft dirt. The UV filter was ruined, the camera was destroyed and the front lens element was not harmed.

Canon EF 70-200/2.8, dropped front first into gravel and sand during a lens change. Lenshood popped off and UV filter scratched beyond use. Front element untouched.

During the time I worked for a newspaper, there were numerous (and since forgotten) other times a UV protected the front element of a lens during times when there were no particular danger around. I threw away a boxful of scratched or broken UV filters. I had a few worn-out lenses during this time but never a scratched element.

These days, I'm a little less cautious. I don't shoot pictures every day and it's usually at a more leisurely pace, so I often remove the UV filter on a lens when I do. I know it will help to prevent flare and may actually make the final photo a little bit sharper. But I still have UV's on all my lenses as a clear lens cap.