I had the same dilemma in the past. My car is parked outside in the street. So i thought of buying a cover for it. For a few days I "religiously" used to cover it every night...BUT, getting the dirty cover off in the morning with your office clothes on is a hassle every morning and my office clothes started becoming dirtier than the cover itself by the end of few days : / the second problem was I cannot cover the car at every stop i make in the day (when i leave the car standing for a couple of hours), hence at the end of the day all the day's dust got accumulated on the car's body and when i covered it, eventually next morning the underside of the cover was also dirty (which became dirtier by repeating the same practice for a couple of days as I could not wash my car everyday before parking it in the street and then put cover on it..) so eventually , accumulation of dust started appearing underneath the covered car... hence I left the practice of covering my car every night as it was more of a hassle and was impractical ...
so, the conclusion i got to was ...either you put the car cover on your car every day after cleaning the car (which was impractical for me) or only to use cover if you have to leave your care covered for a long period of time...otherwise it was more of a hassle putting the dirty cover on and off than a relief : (
