No one has yet really commented on whether image quality takes a hit. I really don't care to do the testing but I was hoping someone more analytical than myself had gone to the great lengths to see if it does make any difference optically.
Personally it is just economics to me. If a lens costs less than the filter, I don't care, I'll just buy another lens. Usually anything less than $100 I don't worry too much about. I did drop a Nikon 200mm F4 smack on a rock at the beach from about 3 ft up and the filter did save the lens, because had it not been on, it surely would have had dented filter threads just like the filter's dent. I have a 50mm F2 with the metal lens hood and never bother to cap it or keep a filter on, but it only cost $25, same goes for the 75-150mm that also cost less than a good filter. I do put a filter on my Rollei 35s, only because it's just asking for finger prints and it gives it a bit more meat to grab for focusing.