I guess the real question is what happens if you do not cover it? Will it get excessive sun, will it get enough rain to overwhelm the gutters, if covered will it trap more moisture than the cover vents can handle and do other covered items that are outdoors gather mold and mildew? Also consider that they are not covered by the factories or the dealer when they sit on their lots.
Here in Indiana, I cover ours because it reduces the ice and snow build up, I park in a pine grove and it keeps the leaves and needle off, we do not have a moister problem but it is often early winter before it is parked.