Different people have different opinions. Take a look at the state of my filters and you'll see why I have them (and why I replace them every 2 years or so). If you're the type to coddle your camera as many are then there's no real point in having them. Take your camera home at night and polish the lens with your expensive lens cleaning fluid and microfibre cloth.
On the other hand if you're like me, and your camera takes a roll through a sandy beach, just grab your sandy tshirt and wipe the front of the filter clean. Camera gets acidic hydrocarbon on it like mine? Wipe the front of your filter with your dirty kevlar / nylon overalls. Animal licks the front of your lens? Wipe in whatever you have on.
Anyone who tells you in absolutes that something is not for protection is talking about themselves and not talking about what may suit you. I for one don't use lenscaps*, don't leave my camera in manual, and don't turn it off. I like the ability to grab and click quickly and get a shot that is quickly missed while others are often getting ready. It takes it's toll on the camera equipment but hey I have a UV filter which protects my lens. Sure I have a lenshood as well but on some lenses like my 10-20mm the lenshood offers as much protection as an overweight midget bouncer with bad asthma.
* I actually have 2 lenses now which don't have lenscaps. I got tired of replacing ones I lost while out shooting.