Ok, so maybe I have a different philosophy.
I returned this summer from a 12,000 mile trip to Alaska and back. No spare tire carried. I went down every bad road not recommended for RVs I could find, such as the road to McCarthy. I was lucky, no flats.
However, this was a strategic decision. I don't really have a good spot to carry a spare and wasn't willing to take up the entire back of my Jeep to carry one. But its not just a spare tire. So many other things can go wrong, from tires to alternators, fuel pumps, injectors, brakes, etc. I talked to one guy last year who spent over a week waiting for replacement shock absorber. The bottom line is that a tire is just one of many things that can go wrong on a trip. I carry spare belts and filters, plus a full tool box and lots of small motorhome repair stuff, but none of that takes much space or dollars.
The bottom line is that you have to simply be willing to have patience if you encounter problems on the road. I was resigned to the fact that a major breakdown could cost me two weeks on the way to or from Alaska. I was resigned to the fact that any one of a number of possible repairs could cost big bucks to fix.
So, a spare tire, more than anything, is really nothing more than a bet on your time and convenience. It is a limited bet, since it happens rarely, is expensive to cover, and the odds are you won't need it. Personally, I would first spend money on a good tire monitoring system. That might save you the actual loss of a tire, as mine did when I had a leaking schrader valve. I caught it early, re-inflated using my pressure tanks (consider adding an air hose outlet and a good air hose), and drove it to the nearest tire shop, which put in the new valve for free. If you catch a flat early, before it goes fully flat, a patch may be all you need.
All that being said, if I had a convenient way to carry one, I would probably do it.
Charlie
2005 Coachmen Sportscoach 380DS Diesel Pusher
2014 Jeep Rubicon Unlimited