All valid reasons on both sides. In the mountains, distant landscapes, and even scenes lit by indirect blue sky, I'll use a Skylight (1B). It cuts through the haze, and I really sort of like the slight warming effect. Also feel less guilty about cleaning it off with spit and a hankie than I would if actually touching the lens ;-) For cases where there is not one of these needs I'll try to remember to take it off. It is important to use a lens hood to prevent as much flare as possible.