An unmounted spare is not that heavy and doesn't take up that much space. It can also save you hours, if you have a flat, especially on a weekend or Holiday. Road service will mount it for you at the side of the road. It usually takes the guy 30-45 minutes to change a tire. If you do not have an unmounted spare, road service has to find one, pick it up, pay an exorbitant price, and then bring it to you and then they still have to mount it on your rim. Depending on where you are, this can take a day or longer to happen. If you always stay close to the interstates, it may only cost you several hours at the side of a busy highway.