Originally posted by SpecialK That is indeed a special effect - modifying white balance. But not protection.
Never thought of that as a special effect before, but I see your point.
For purely protective purposes, I never bother unless 3 conditions are met:
1. It's an unusually expensive lens and or/rare enough that it would be very difficult to replace.
2. I'm using the lens where there is some danger to it in the environment (sand, dirt, folks jostling around)
3. The filter is of the highest quality and fully multi-coated.
Otherwise, a sturdy hood does just fine for protection.