BTW - I read somewhere that tires that aren't driven on crack and rot faster than tires that are driven on.
This because tires flex when driven and that (allegedly) works fresh protective oils, waxes, and UV protectants to the surface of the tread and sidewalls, which (allegedly) slows decay from ozone and UV.
So if that's true, driving on a tire delays decay/rot/cracks.
Even if that's not true, I guarantee an unused spare will eventually rot/crack and be wasted (regardless of the exact causes).
So if you don't rotate your spare in, it will eventually go to waste from cracks/rot, AND the other 4 tires tread will wear out 25% faster.
Warning: I often edit my posts a few times to get them complete or correct errors.