Been using covers forever, not sure, probably since the 70's. In carports, in garages and especially (mostly) outdoors in Phoenix sunizona. The cover is for the sun. All other benefits are on the side, security, dust, bird airstrikes etc. Even my cars in the garage get a sheet of plastic over the carcovers, since the dust still finds it's way through.
I find that if I want the car to stay clean in the rain, another tarp or waterproof fabric must be put over the cover and attached in such a way as to weather a hurricane. It probably wouldn't, but you have to think that way. You WILL damage the paint if it gets wet underneath and/or if it gets the bellows effect 'thing' going in the wind. If mine's clean and I have the time before the rain starts, it gets a sheet of painters plastic over the cover and then a heavier older cover over it to hold it in place. About 15-20 min of prep to prevent a 3 hour wash and possibly other issues. One tip: if I ever could give one is that the car has to be clean when covered, and mine gets a kozak + window wash or a sponge bath every time its driven.
"The cover is only as good as the 'DIRTIEST time' it was put on the car". So, I never use it on a dirty car, which is to say if the car has been driven more than just a few miles, it's dirty.
