Originally Posted by
Slash5
Even for road biking, I'd think you could get a slurry of water and road dirt down the stem. A presta valve is nothing but a threaded shaft with a rubber seal at the bottom. Seals work better if they are clean.
Actually they twist closed so water doesn't get in. Even left completely open, I doubt water gets down there anyway. But if you like covers, the colored ones are classy.