IMO outdoor covers are a tough one. They need to be breathable and the car needs to be clean whenever the cover is on. I personally never cover anything outside unless it has roof/weatherstripping issues.
I am always looking at cars for sale and I've seen plenty of people with the best intentions of saving their car just do more damage with the cover than if they would have just left it uncovered. My favorite is when I try to buy a car that someone is "saving" by parking it on dirt and covering it with a 10 dollar tarp from homedepot. At least in the Midwest that might be the worst thing you could do. You would be better off parking the car on that tarp then covering it with a plastic tarp.