I think having an unmounted spare is a reasonable measure and wish that the car design accommodated a tire mounted underneath. This is a luxury car and it's reasonable we pay for the expensive tire but not the expensive 2-day stay in a hotel waiting for a tire delivery. As for the kit, I want to drive the car not be it's mechanic. That was for when I was in High School. Again, if there were a sling or something, I could bring the bad tire home and fix it in the "comfort" of my garage instead of the side of the road.
Come to think of it, an unmounted repaired RFT might serve nicely (and be more green) for this "travel spare" approach.