And the perpetual debate continues...
In all of photography, I can't think of a subject where so much has been written about such a minor issue. Personally, I use protective filters only in adverse environmental conditions, but doesn't mean that's right for anyone else.
Lens hoods, filters, and insurance each provide overlapping but different types of protection. A lens hood can deflect some incoming dangers, assuming they come in at the correct angle. They can also absorb some energy from a drop, and act as an "early warning" system when you are about to strike an obstacle. But when grit from a dirt bike or salt spray is coming at you, the lens hood isn't nearly as useful. Both hoods and filters work best against small "day-to-day" traumas, but for gear-destroying events, "all risk" insurance is your best bet.
I've personally owned two lenses where I've picked up "cleaning marks" on the front element which a filter may well have prevented but a lens hood did not. The effect on lens performance of the marks is negligible; the effect on resale value much less so. One lens I sold at about a $100 discount off of a copy with clean glass. The other, a Nikon 180mm f/2.8D, went begging at the same reduction in price. Nor does insurance cover such losses; it's rightly considered normal "wear and tear."
On the other hand, quality filters are not inexpensive, and adding even the best filter increases the likelihood of flare spoiling your shots. Compared to a clear protective filter, UV filters also block a small amount of additional visible light in the violet range. My tests showed that adding an B+W MRC 010 reduces overall transmission by about 1.8% and imposes a color shift of about 25 degrees K. These aren't huge effects, but there's no compensating upside unless you:
(I do both, which is one reason I haven't swapped out my 010 MRC's).
- shoot either non-chromogenic B&W film or tungsten balanced color film AND
- shoot either 25+ year old lenses or simple Tessar-like lenses.
In the end, it's your gear, your shots, and your money. If you feel uncomfortable enough using a lens without a filter that you start missing shots, then by all means use them. If, like me, you're willing to accept that lenses are tools, and that cosmetic wear and tear is part of the deal, then don't. No matter how long people argue about it, there's simply no "one size fits all" answer.