pros:
1. you can keep most of the snow out of the tire/wheel
2. may preserve the tire longer from dry rot and elements better than a exposed spare
cons:
1. what snow you do get inside will stay inside since the cover will hold melted snow and may refreeze and stretch/rip the cover over time
2. you may lose the cover
3. the backside is already exposed w/o a cover so mute point since you wont see it if it is used
4. the way the spare is mounted you will get some dirt, etc on the face of the tire but nothing a decent cleaning will not clear up (cover may lessen this but the location spare is will not eliminate it)
5. the cover being exposed to underside rocks, etc may damage the cover faster than normal which may result in replacing it more than once and cancels out the reason you got a cover
just some things to think about