Personally, I have B+W MRC UV filters on all my lenses (F-Pro mounts on most, XS-Pro mounts on the wide lenses). One reason is what Chris mentioned - for 'weather-sealed' L-series lenses except the supertelephoto lenses, a filter is required to complete the weather seal. I also find that the B+W MRC coating gets less dirty and is easier to clean than the optical coating on the front element of most lenses (which is why Canon is promoting the fluorine coating on their newest lenses as being smudge-resistant and easy to clean). Also, if get a fingerprint or spray of water in the middle of shooting, you can simpy unscrew the UV filter and keep shooting, and that's a lot quicker than cleaning the front element and missing shots.