Buying a used tire to carry as a spare?

When we traveled to Mexico with a Tracks tour years ago I purchased a used 22.5 matching unmounted tire to use as a spare. My friend uses the same size, so he purchased the serpentine belt to have on hand. We used neither...thank goodness. I traveled around with that hunk of rubber in the bay for years, thankfully never needing it. If I did need it, I would have had roadside service mount an outdated tire, at my expense, which I would use to drive to the nearest city, then purchase a new tire and have it remounted at my expense. With the current roadside policies offering to find a tire dealer to bring the proper tire, or tow you to a tire dealer, it seems a bit silly to carry a spare that is probably beyond the owners capability to mount themselves. I just plan on purchasing new tires every 6 years or so and hope for no road hazards.
JMHO