UV filter vs no UV filter. This is a debate among photographers of all levels of experience. Technically, what does a UV filter do? Well it filters out UV rays to some extent which may make your images look a little better, although I have yet to see it. So why would anyone put a filter on a lens when it's effects are going to be negligable? To protect the lens.
A couple of years ago, I invested in a 120-400mm zoom telephoto lens ... right before we went to Hawaii on vacation with friends, and I had great hopes of shooting surfers and kite surfers off of Maui. After being collected at the airport, we began the vacation with a wonderful lunch at Mama's Fish House right on the beach. Not knowing what we would see from the terrace, I put the long lens on my camera and brought it to lunch with us. To keep it out of the way, Peter perched it on a ledge near an open window and what do you think happened? You're right, it got jostled and fell to the floor much to my horror. I couldn't breath - the lens wasn't insured and it also wasn't even paid for yet. Well, the lens came through unscathed, but the UV filter that was on it was badly dented and the glass was cracked. It cost me a wopping $25.00 to replace that UV filter, which you can bet I did.
Another time, my husband and I were shooting a Mini Cooper Road Ralley in the Hamptons. He was shooting stills of the cars and participants with his trusty Canon 30D as he was also filming the action with a video camera mounted on a tripod. He turned away from the tripod for a single moment and as he did a gust of wind took his video camera down for the count. Once again we were horrified and terrified about the possible expense of such an accident but ... once again the UV filter on the camera saved the day, it was destroyed but the lens was perfectly fine.
The moral of this story should be clear - put a UV filter on every lens/camera you own - it's the least expensive insurance policy you'll ever buy.