I would sooner drive through the States of New Jersey or Massachusetts with a firearm openly displayed in my car than clean a rifle without first putting lens caps on the scope. The former will only get you arrested or shot, but cleaning your gun with its scope lenses exposed can wreck a sight, or cripple it for life.
Scope lens coatings—microscopically thin layers of stuff like magnesium fluoride—are as important as the glass itself. What they do is prevent lens-to-lens reflection, and thereby vastly increase light transmission, color contrast, and all sorts of other good stuff. If you’d like to see what a major role they play, look through a binocular or scope that was made before lens coating came into use, and you will be appalled, probably emitting a small shriek of horror and flinging the offending instrument aside as though it were a venomous serpent—say, a fer-de-lance, a Russell’s viper, or a banded krait.
Lens coatings have gotten much tougher in the last 20 years, and now cause water to bead up, letting you view in a downpour, and are resistant to abrasion (within limits) which they were not in days of yore. But they will not withstand powder solvent, or ammonia-based copper killer, or oil, or any of the stuff that we use to clean our bores. If you get those chemicals on a lens, you’ll get certain damage even if you get it off immediately, and the odds are you won’t see it right away, so it will have plenty of time to work.
The most common way this happens is when you put solvent on a phosphor-bronze brush and shove it through the bore. When said brush emerges from the muzzle, it gives off a mist of solvent. Sometimes the droplets fly back with enough force to hit the lens directly; or they may float back, borne on the air like a really dire fart during boarding-school religious services.
Either way, your scope is screwed and so are you. You may clean it off, but things are never going to be the same.
“Tough” is a relative term, especially when applied to an optical lens. Here are some other things you should never do:
Wipe the lenses with your snotty bandana. God knows where it’s been and what’s on it.
Wipe off dust with same bandana and a gob of spit. It’s like wiping lens-grinding compound into the glass, and that’s been done already, thanks. To get dust off, you use a lens brush, and flick at the dirt, gently.
I own about a dozen Butler Creek Bikini scope covers, which I’ve distributed everywhere I might have a rifle. One size fits all scopes, pretty much; they give you a good seal; and they’re effective in rain or snow. And they’re a hell of a lot cheaper than a new scope.