I used a car cover (OEM Mini) for a couple of years and will never use one again. It's impossible to keep dust out from under there - small dust, large dust, you name it. Then when the cover moves from the wind, the paint gets scratched. Also, if the car is going to sit for any extended time, the cover will hold in moisture.
Perhaps the above experience was because I had a poor car cover. Maybe there are better ones out there that don't scratch the paint. Either way, I was very anal about only putting the cover on a clean car/etc... but it still didn't help.
mb