I admit it, I've been riding fenderless this week during our California rainstorms. The reason: my fendered rainbike is out of action with a broken brake cable. Yes, I should fix it, but it's just so much easier to grab another bike, and it really hasn't rained that hard (until today). My raingear has kept me dry, but I'm sure the spray is annoying to following cyclists, and spray from the front tire hits my face.
Here in the Bay Area, even the recreational road cyclists have fenders on their bikes. The Guardian has a humorous piece shooting down the usual excuses people don't use fenders.
See also Velo Flaneur's Southern California Fender Primer.