Hi Bob,
Hope you got to go to Walcott, IA and see the Truck Show this year.
Anyways, to answer your question, spare tire for drive and trailer tires. 90% of the blow outs we have out on the road happen were just the tire itself is lost. Most companies do this because of cost and it's an easy fix, 15 min. to replace.
It's getting someone out to where you're at that's the pain, Arizona being one area, on I-40. There are the rare happenings that we loose the whole tire and the rim, I.E., Accident with another truck, car, wall, etc. Having a tire on the rim would be stolen quicker also.
One rim can run up to $500.00 or more, then add the $150.00 for a new tire, adds up when look at it. It costs $3500.00 +/- just to put new tires on a tractor, not including rims.
And you thought this hobby was expensive............
HTH.