To filter, or not to filter, that is the question:

I will use a UV filter as a protective measure under certain circumstances. For example, doing aerial shots and sticking the camera out of the open door or window of the plane won't allow the lens hood (too much wind causing shake) but I use a UV filter to protect from potential "sand blasting" from dust or insects. For a landscape on a tripod, no (protective) filter but always a lens hood except that this obviously doesn't apply to landscapes using a ND or ND grad.

For most purposes, the hood is adequate protection. I seem to recall though, that Canon's water resistant "L" lenses require a threaded filter to qualify as water resistant.

My experience has been that the absolute image quality is better sans the filter. YMMV, but if you buy protective filters, get good ones.