Tire covers? Are you referring to the hub cap or an actual cover for tire storage?
Valid point but overall you should always try to keep your car out of the sun because it have a similar impact on the paint, rubber seals, etc. Unless you're leasing this and don't care much about what will happen 5-6+ years down the road.I'd go with a cover even if it makes the rear look disproportionate, more for protection from the elements than anything. Why submit the tire to sun, rain and snow which harden the rubber faster?
No matter whether you attribute the costly damage to negligence or simple judgement error of room when backing up, when the Bumper offers (zero) Protection an your Rear Mounted Tire accentually becomes your Bumper, it doesn't take a genius to figure out that just a minor "tap" is going to cost Thousands of dollars in the repair.To be honest that's more of your own negligence than anything else. Maybe for someone having this setup for the first time might require some time to adjust but overall if you're careful enough there wont be an issue. The pros far outweigh the cons. Plus how often do you need to access the rear cargo area?