Not sure about the writer on this thread who asserts that you can replace a front lens element for the cost of a good filter. My mail is delivered to a different planet. Ditto for the gentleman who claims that a lens hood is all the protection a lens needs. (I agree it's the first and best line of defense for the knocks, bumps and other physical slings and arrows of outrageous fortune, but my world is sure a lot dirtier than his.) Try as I will I can't keep grease smudges, grit, condensation spots, etc., off the front of my lenses. And who hasn't had a lens cap--especially a Canon lens cap--fall off inside the camera bag, a presumed safe haven for lenses?
And that to me is the value of a good filter. Glass has an affinity for dirt. You gave up smoking and surprise, surprise, the inside of your windshield still gets that ugly film on it just from being exposed to the atmosphere. While I'm always amazed at the number of people who tote expensive DSLR bodies with horribly smudged lenses, most folks who are serious about photography want to shoot through clean glass. If you do, some part of the optical system has to get cleaned. And as they say, it's not the fall that kills you, it's the landing. If you can live with the dirt all over the business end of your camera, it won't damage your lens (unless it's caustic or arrives at sandblasting speed). It's the process of removing it that is at least mildly invasive and abrasive. Aside from the grit that's currently on the lens, there's all that residual stuff from previous cleanings. (It didn't just evaporate from your microfiber cloth, you know; in fact that clingy cloth probably picked up some additional material from the inside of your camera bag.) No matter how careful you are, it all gets dragged across the surface of whatever glass you're cleaning, and it all nano-etches. No one is claiming that a UV filter sharpens a lens, but neither would anyone claim that years of cleaning the front element improves a lens's optical performance.
I've had a couple of my lenses for seven years, and in that time I've cleaned the filters more than a hundred times. I plan to keep most of my lenses for the rest of my life, and I can foresee a possible need to replace filters at some point. What I won't ever have to do is worry about degraded performance from the lens elements themselves. The filter in front protects the lens not only from particular kinds of dirt, but almost eliminates general atmospheric contamination. I know that the surfaces beneath the filter are in pristine condition, because except for using a puff brush to remove the occasional mote of dust that sneaks in during a filter change, those elements, after an initial personal inspection, have never been touched, much less cleaned.
And for all you gonzo types who think filters are a collaborative scam of camera salesmen, here's a question to answer honestly: who would you really rather buy a five-year-old lens from: someone (like me) who obsesses a bit about protecting the exposed elements of his lenses, or someone like you?
Finally, it's not as if mounting a filter is an inconvenience, and for me the cost of a top-of-the-line filter is trivial, not because I have an unlimited budget, but because I have a very tight one. When I do take the plunge for a lens, it's for the "expensive as L" variety, and it's a lifetime investment. And if I've spent $1700 for a lens, why wouldn't I spend another $80 to maintain and optimize it's performance?