If you're in a particularly hostile environment … motorcycle scrambles, seaside/beach, just filthy weather, a piece of glass that can be wiped clean of dust, grit and spray without risking damage to the underlying lens can be at least reassuring, but only leave it on for as long as necessary
Other than that, ND, polarizing (and infra-red, if you're interested) are what I carry for use when needed for effect.
I've proved to my own satisfaction that some UV/Skylight filters do cause unwanted flaring/reflections on some lenses under some conditions.
For "protection", from impact and stray light, I invariably fit a lens hood, usually the dedicated bayonet-fit unit which lives reverse mounted on the lens when it's in the bag, but any hood that's not so long as to cause vignetting is better than none at all!