Even though a good DP will have a port for it's on board compressor of at least 110psi, carrying a spare isn't about changing it out yourself, as many threads will attest to, so won't go into all the pros concerning this subject. As for the cons, it's just a matter of space for something you may not ever need and no matter how hard I tried, carrying an unmounted tire is a real chore, but mostly because of the storage loss and again for something you may never need. With my old coach, it wasn't a problem and always mounted, in it's own compartment.
I can say this and with a blowout you can sometimes be at the mercy of your roadside assistance, when you could otherwise just have them change it out for the insurance and then go and do your own shopping around, depending on how old the tires are. Then there's the possibility that the affected tire is still new and under warranty and needing an inspection by an approved dealer.