I am inclined to use a UV, Skylight, or clear filter to protect the big front element of some lenses, as sometimes those are made of exotic glass that is a bit softer and more scratch prone than traditional crown or flint glass. As to a filter adding surfaces to clean or degrading IQ because of additional glass: 1) on any lens you clean the front-most surface of the first element which becomes the front of the filter - the filter will keep dust & finger smudges off the foremost lens surface; 2) filter glass is very thin and very flat and should not degrade IQ anywhere near as much as the multiple much thicker glass in a TC and how many of us are willing to use one of those?
Once when moving about to get a better angle on some wildlife the tip of a dead branch struck the front element of the tele I was using, sticking in down the full length of the lens hood. Careless of me, but when preoccupied with getting a wildlife shot you don't always think about "take off the hood, put on the cap, move, remove the cap, replace the hood." I suppose one of those soft bag-type caps that go over a hood would spared me an Adrenalin surge, but does that mean I'd need two caps, a soft one when the hood is on, a hard when when the hood is off/reversed because a soft one might be pushed in to rub against the lens if it were put on when the hood was off/retracted/reversed..
SIGH! Why aren't there quick, simple. final satisfactory answers instead of one question leads to another and answers that are always compromises?