I cover my tires pretty much anytime they are not rolling down the interstate!
UV rays kill tires. A set of tire covers will definitely extend the tire life. I will give you an interesting comparision. My father and I both bought new trucks at basically the same time. (within a few months of each other) They had the exact same tires and the date codes on the sidewalls where within 6 weeks of each other. My truck sat in the garage (and out of the UV rays) when not is use and dad's sat outside. He had to put new tires on at about 40K because the sidewalls were all checked and cracked. When he put the new ones on I looked at mine because I had about the same mileage on mine and my tires looked perfect. Covers will definitely help extend the life.