That's the main thing, for me. Even my road bikes get dirty though, and wheels and valves pick up a layer of dirt and contaminant, even riding paved roads. And I don't always clean my wheels - most of the time I clean and lube the drivetrain and then ride. I like to know that when I do need to pump up the tire, I can just unscrew the little cap and have a nice, clean valve. It keeps the inside of your pump head cleaner too since you're not sticking dirty valve stems into it all the time.

Cosmetically, seeing a bike without valve caps is like seeing a car that's missing its hubcaps. Sure, they're not functional, but the car still looks ghetto.