All my camera lenses have a high quality multi coated clear lens protector on them. Why? These protect the expensive glass on the front of the lens from dirt, dust, water and from scratching. They seem easier to clean as they are flat and much cheaper to replace if they do get scratched or broken.
Some people think there is no need for lens protectors as the lens hood should protect the lens from being scratched. Others think why put an extra piece of glass in front of a lens.
One time I was photographing in Egypt with my camera on my tripod with my lens hood on when a rogue donkey came running from nowhere crashing into my tripod. The camera crashed to the ground lens face down on gritty sandy earth. The filter was written off, the £1,800 lens was ok. This wasn`t the only time that a lens protector had saved one of my expensive lenses.
As for the arguement that they degrades image quality then I would have to say that all my published and exhibited images have all had some kind of lens protector on, including the front cover shot of National Geographic.