The only time a cover provides some protection is when it is new. Once the cover has been taken off, or has been in use for a short time, it will start to get dirt that will accumulate on the inside of the cover. This dirt will act like sand paper and when the cover is moved by the wind, it will eventually sand away the paint surface.
Even in my coastal Florida home, I will not use a cover and if the manufacturer used a good quality paint, it should last for a long time exposed to the elements.
On the other hand, I do enjoy watching my neighbor struggle whenever he puts on or takes off his cover. I always wonder when the cover will win. :B