I've had one pair of glasses for 2 years and I clean them multiple times a day with my shirt. I frequently go hiking in dusty conditions, tumble in a gym with them on (they fly off and hit things), etc. After those 2 years of use they still have no scratches at all on them and they're a cheap pair of glasses. I'd like to think that if my glasses can handle it, the front element can handle just as much abuse.
If I were to go and shoot a rally or rodeo then sure, I'd toss a filter on there just for the peace of mind, but for daily use I just don't get the point. I've seen a definite flare issue caused by a UV filter, after that they stopped going on my lenses. Not only that, I personally find that a filter is actually harder to wipe clean then a lens is (unless it's sea spray, because with a filter I'll scrub that sucker as rough as I feel I need to, I'm a little more gentle with my front elements).