For protection against knocks etc. a lens hood probably does a better job than a filter. Carrying a camera with lens inward as EarlJam said is another way and something I always do. If you're concerned about the cost of replacing a lens if it becomes damaged, get insurance. You may already have it as part of some other insurance like house contents insurance. But in 50 years of photography, I've only damaged one lens and no filter was ever going to save that.
As for preventing the need for cleaning, most lenses don't need frequent cleaning. A blast of air is all that's required. A bit of dust on a lens is likely to have less effect on the result than the filter you put there to stop it. And, if you do need to clean a lens, with the correct materials and cleaning fluids, it should have no lasting effect on the lens.
I wonder how many people who habitually put protection filters on their camera lenses fail to do so for their most valuable lenses - their eyes.