OK...some think it can't happen to them.
Dusty parking lot, strong gusts prior to a thunderstorm....grit blows on boats near parking lot...rain makes grit stick to wet fender cover (granted not all fabrics this may happen to...but the cotton, fleecy ones are suspect), strong gusty winds during thunderstorm means boat/wave action (but not a lot of torrential rain to clean the covers), scratches result.
Seen it happen...so all I can say is ....even though you may never have had a problem...it has caught good, experienced boaters.
Just like a bare fender can have stuff on it....say from a fueling situation and there wasn't enough time to clean it before you got to your slip for whatever reason and got forgotten till scratches occur.
I never said covers were better or worse...just don't think that because of them you'll never get scratched....no matter how meticulous you are unless you're the type to tie up a fuel dock cleaning fenders while others circle....