Put me on the side of pro-cover. My home is in a wooded area and have my MM parked on a pad next to my house. The reason(s) I cover it are many...
Spring: Pollen can be very heavy at times. After it rains (and dries), the pollen leaves a hard crusty residue that gets into every single crevice that only a power washer can remove. The cover is an awesome barrier and keep everything clean as I left it.
Summer: I don't cover it. My rig is under shade 98% of the time and I'm usually doing mods anyway.
Autumn/Winter: Leaves are falling along with the acorns. I cover the plastic vents and AC cover with bubble wrap and then the cover on top. They call dropping acorns "widowmakers" for a reason!
Finally, the ADCO cover I have is aesthetically pleasing as the color scheme blends in nicely with my surroundings. It takes less than an hour to put on. When I take it off, it's clean and ready to roll.
The prices here are a little high but you can see the color scheme. I got mine on Amazon as "used" item (it wasn't) for $110. Pretty good bargain.