I agree with Capriracer on this one, let me add another reason not to run them in the summer.. Rubber hardens when used in the summer, soft tires heat more....., Here was my first experience:
I bought two pairs of the original Sears Ice and Snow tires (made by Armstrong) and put them on two nearly identical cars (Fairmont/Zephyr. The Zephyr a wagon and was used for long trips to the mountains for skiing. The Fairmont was used around town a lot, and often towed a boat. I elected to leave the snow tires on year around on the Fairmont.
Now I was orginally stunned by how much better these ice rated tires were then any previous snow tire I had owned.
The Zephyr tires got long distances, but winter only use, other tires for summer. The Fairmont went shorter distances, but year around use. End result, both sets wore out in 4 winters BUT:
The winter only use tires still had incredible traction as they approached bald - on either car- they could pull a steep hill on totally snowpacked streets, and the rubber was still soft to the fingernail.
The year around use tires on the Fairmont had become rock hard, also in only 4 years. They were as bad on snow as any bald standard tire. We had noticed this effect by the second year of these tires lives, but just thought it was our imagination.
Incidentally, the Zephyr - with the nearly bald winter use only ice tires - was used to pull a ski boat to a waterski Show on Jan 1st (yeah, I know, skiers are nuts, but it was nationally publicized)! It easily pulled the ski boat up the snowpacked hill leading away from the river, while many FWD cars, pickups, etc could not drive up it pulling nothing!
I bet CapriRacer can tell us lots more about the relevant rubber chemistry issues as we age these tires in a hot environment!!