I recently read an article (I don't remember where) that said valve caps are necessary for schrader valves because dirt and dust can foul the mechanism, but caps aren't necessary on presta valves because they seal themselves when screwed down properly. I use caps so the presta valves don't rip my jersey when I fix a flat and take the bad tube home for repair or disposal. I also think it looks better, but that's just me.
Yes, that is true, the valve core in a Schrader is subject to damage and or clogging due to dirt and water, and that crap can also make the valve leak if you don't keep the valve cap on.
Now there is possible same situation with the Presta as well, but most people do not use the cap because those valve just don't seem to care as much as Schrader does, like I said earlier if you're going to be in a wet and or muddy environment you should use the presta cap, and the schrader cap. Most roadies don't ride in the rain so most roadies ditch the presta caps, but my touring bike came with Presta valves so on that bike I use the cap, but not on my regular bikes.