As Max mentioned, the tires age and can be of little use after being the spare for years. I presently have a tire in my garage that was brand new, came with my old Blazer and was not once used. When the car was 9 year old I had a flat and had to put on the spare. Drove for 1000 miles and when I was within 20 miles of home, late at night at 75mph, I hear a terrible noise that sounded like the whole vehicle exploded. Then it got quiet again and I never stopped. When I pulled into my garage I looked at my tires, the used once spare had no tread on it, the entire tread area had peeled off the tire and left the steel belts. The reason I kept it was to remind me to check my tires and because it got me home alive, never lost air. The original tires, just like the spare, had gone 85,000 miles with only one flat, so it was an excellent tire, it simply dried out over those 9 year in the back of the car.