OK--lets talk "Apples to Apples" --we are not talking light duty truck tires here. Carrying even an unmounted 22.5 [semi-truck] tire is going take up a lot of space. If you follow the 5-6 year service rule on changing out tires, you probably wont have a blow-out due to age [road hazard yes--age no]. If you do carry a spare, it wont match a dual [due to age/wear] but perhaps could be used on the front. Maybe only in "doggie-years" but your spare will continue to age while being stored so you are not putting a "new" tire on anyway. Finally, if you do blow a tire on a class-a, it brings into question the status of the remaining tires--accordingly you may be looking at new tires all the way around. Assuming you have a good travel/tow service, you can negotiate an acceptable replacement tire until you decide what to do about all the tires on your rig. My experience is that you can get a good temp replacement for $300 and expect to get half or more back when you trade it in as slightly used. Therefore, I cant see carrying a spare--best case, I can maybe save $150 someday, and worst case, I have a $600 old tire in my basement that I never used--IMHO.
2003 40' MDTS
Garden Ridge, Texas