A reason for carrying an unmounted tire in a place like Baja (and other parts of Latin America and Africa), is that it easy to find a shop that can mount your tire, and hard to find a shop that has the right size tire.
Even in parts of the USA it can be hard to find certain sizes of tires. The Element, for example, has a stock 215/70/16 tire, which falls in the crack between large car and small SUV sizes. I know of one Element owner that had to run all around the Cedar City, UT area on his spare before finding a replacement.
When I drove to Alaska some years ago, the guide books were recommending taking along an inner tube, on the theory that a local shop could install that in an unpatchable tire in a pinch (and a replacement size was not available). With relatively new 235/75/15 tires I didn't have occasion to test that idea.
paulj