Max, I've never seen a flat spot on a tire but then again my vehicles never sit for more than 2-3 months in one place without being used, so I'll defer to you on that one.

Arney, I'm not sure why spares last longer but I suspect that car/truck tires, including spares, have a different composition than trailer/RV tires. Add to that the fact that car spares are usually kept in the trunk out of the elements, especially direct sunlight.