I have a $95 UV(Haze) Kenko Zéta on all my lenses (alternates with B+W Kausmann C-POLs). I have never, not ever put a $5.00 filter on a $3,000 lens, as much and often as I observe others doing that. It's much easier to wreck a reasonably expensive filter than to wreck the front element of a lens, that's for sure. That is the guiding philosophy of providing protection, if not function (but a UV filter will reduce the excessive blue at higher altitudes; for everyday use though it has little visible effect and should be there for the protection of the front of the lens, no less). Filters do introduce an additional risk of flare and ghosting that can occur more easily, particularly with aspherical or apochromatic front elements.