As I said in a different thread on the same subject;
Carry a spare either mounted or unmounted
Mounted means that anyone roadside assistance dispatches should be able to get you going quickly. I have had roadside assistance in Tennessee be nothing more than a big huge guy with a hydraulic bottle jack and a big breaker bar. He probably could not have mounted my 19.5 without using starting fluid. On the road quickly as I let him use my Electric impact wrench.
Unmounted means that most people dispatched will be able to get you going. However in my case above I believe I would have been waiting for a better equipped roadside assistance person.
No spare means that roadside assistance will have to find someone to dispatch that has the size tire you need and that may take hours if you are in a remote area or on a back road. Or you may have to wait until the tire shop that has your size opens up in the A.M.
Will the tire be the brand you prefer? Who knows.
Will the date code be recent? Who knows.
Will you be able to dicker on the price. Come on you know the answer to this one.
In addition you will have the joy of getting a tire that is not balanced when you start your journey again. I prefer balancing to the packs they throw in to balance but with the packs until you have gone some miles your rig will feel different