A spare tire for your TT is one of the simplest forms of insurance against an aggravating side-of-the-road breakdown, and since most trailers I've seen come with a spare, it would seem foolish not to keep it with you and maintain it. It's akin to going on a road trip without money or credit cards, thinking you'll have enough gas and food (and other things) to get back. Some people I've heard from even place a monitor on their spare when they have a tire monitoring system for all other tires, so they can see if there is a problem with it.