I dunno... I have some reasonably nice glass but keep Hoya HMC UV(C) filters on pretty much everything. While they may not necessarily be the greatest filters with the least impact on image quality, they have saved me a couple times. I'd like to think that using the filters out here in the desert helps to prevent my front elements from getting sandblasted (car windshields look pretty bad after about 2 years out here). Realistically, though, the times they've saved me are the times I've used them like helmet tear-offs (lens gets seriously gunked for one reason or another and there isn't time to clean it, unscrew the filter, chuck it into the camera bag, and keep shooting).
I do waffle back and forth on whether I should have them on sometimes, though, since they do cause a slight contrast loss.