I'm a biker; law says I have to wear a crash hat whilst riding. Crash hats usually have a visor to keep the wind and bugs out of your face, and they are usually made of lexan perspex. Depending on how many miles, and in what weather one rides, how long a visor might last before its so 'etched' as to need replacing is a pretty big variable, but, for the limited miles I do, I get around 3-years out of one; higher mile every-day riders probably need a couple a year... Oh-Kay...
More damage is probably done to a lens front element.... or motorcycle visor, NOT by the stuff that is chucked at it 'in use'... but in the cleaning regime to remove it after.
I can be riding along and ride into the hail-storm of stones, grit, sand and general crud falling out the back of a ballast lorry, at maybe 60mph or more.... THAT doesn't tend to abraid my crash-hat visor very much; actually takes QUITE a hefty hit from something pretty sharp to actually put a notch in the plastic, and that will if it occurs tend to be pretty minuscule. What WILL do more significant damage to a motorbike visor, is when you grab a damp cloth and rub that across the surface, and any hard granular crud stuck to the surface, gets stuck to the damp cloth, and dragged across the plastic, under pressure, as you 'rub'... which is why I tend not to... I wait until I get home, or find a convenient rest room; fill a sink ad 'soak' the visor, then wash under the tap, to get rid of as much as possible, with the lightest possibly 'action' on the surface.
IE: it is far more likely that IF you are going to damage a lenses front element, YOU will damage it in your cleaning regime, than any air-born debris will 'in use'.....
And lets be a little realistic here! Just like the crash-hat on my motorbike, I am under no illusion will 'save' my head f I crash hard-enough.. Its not a magical invulnerability field FFS! If I come off, its still gonna HURT! and I'm still likely to get deaded! A filter on the front of my camera, then, if the lens takes a smack hard enough to break it, PROBABLY isn't going to save the lens! Just like the motorcycle hat, it may save a bit of hurt, but? Who knows! Lens takes that sort of hit, front element will be lucky to get away with it, but,that's not the only damage that can occur, and whether any of the internal elements or mechanism have been jogged, ad the focus is gog to stay in limits let alone calibration, would be of as much concern as a chip on the glass!
Here's one for you, since both 'beach' environments and cinder speed-way have bee mentioned...
There you go, Weston-Beach-Race about twenty years ago! Look at that rooster tail off that bikes back wheel, beg chucked at the rider behind, and ME as he went through! THAT is actually ot likely to do much damage to my camera! UNTIL I come to wipe the muck off!
Oh well, how many people take photo's of beach races! Or speed way, or n beaches, or or or.. yeah! Such extreme examples of the photo-environment ARE pretty easy to diminish BUT, rather immaterial; doesn't matter WHAT put the much on the front of the lens, if it's the getting it off that does the damage... and more, you could have a perfectly clean lens, and do damage to it transferring grit from your last dirty environment escapade onto it again, when you pick up the micro-fiber cloth!
Now, motorcycle visors are as said, usually poly-carbonate plastic, not glass, they do scratch and more easily; BUT like a lot of modern lenses, to do actually have similar 'anti-glare' coatings and the like and damaging a coating layer is just as detrimental on these as on lenses, and likely with far less pressure or mechanical abrasion than on the actual plastic or glass; more still, once a coating layer has been damaged, it will likely start to deteriorate, and grow.
SO.. ultimately comes down to compromises... and cleaning..... how harsh the photo environment may or may not be, is not that big a deal; how diligent/careful you are about cleaning far more so. Do you use disposable wipes? Do you always use a brand new micro-fiber cloth? Do you use a rocket blower or compressed air can? How hard do you rub?
A-N-D this 'sharpness' thing... so often NOT actually a quality endowed by the lens, but subject lighting and reproduction 'contrast', focus control etc etc etc.. WHICH I have to say is another can of worms completely, and sorry, but IF clinical sharpness is the most critical thing to your photography and what you judge a photo by.. carry on, 'cos, well, you will likely never be convinced that 'other' aspects of a photo, like whether there is any interest in it to any-one, MIGHT also be important, let alone, possibly more so! It is an irksome argument, like chefs arguing that a stainless, dish-washer safe, Sabatier knife, will never hold an 'edge' like a tempered steel one! Which may or may not be more or less true, BUT CHOP THE EFFING ONIONS MATE! I am hungry! It wont make THAT much of any difference to the chuffing meal! Just how long you spend faffing with the strop and olive oil!
So, do you risk the more delicate coatings of a modern lens against your cleaning, OR risk some small perceived sharpness loss, against a UV or protection filter, that IF you skuff up isn't too big a deal to chuck away, and could be removed in those few instances when any effect it may have on perceived sharpness, might, make some critical difference to the overall image?
Personally, I will leave the protection filters on my lens, and not have pangs of fret when pull out the cotton buds to clean; and not risk poking holes in the coatings on the front element; and live with any minimal IQ degradation, in those instances it MAY case some, and I could take it off! B-U-T most likely cause of any less than satisfactory image will almost certainly be me be down to any of the gazillion other factors, mostly ME, not the camera, not the settings, not the lens, not the focus scheme, not the filter on the front. MOST likely to be the cause of any less than stunning picture!
REALLY, of ALL the stuff, that could give cause for criticism in my photos? Having a UV filter on the front of most of my lenses, REALLY isn't going to make barely a hapeth of difference to the 'standard' of my results!!! It REALLY isn't! DOES make me feel a little more comfy, when it starts raining, and I have to wipe the front with a tissue! But, different strokes for different folks, IF you are more comfy, wiping the front element, and confident you aren't doing more harm than good, AND that t makes Sooooo much difference to the quality of your photo's... carry on.... obviously your skill is such that everything you shoot is of such standard such minimal differences are of crucial importance to the perfection you obviously not only strive for, but must regularly achieve.....
(For Note: Weston-Beach-Race shot, was taken without a 'protection' filter; ISTR it was shot with a fish-eye, that cant take one.. gives clue how close I must have bee to that hail of sand off that chaps back tyre! And it's not exactly improved any by any enhanced 'sharpness' is it?!)
