When I bought my Sailuns I bought 7 (triple axle) for a few reasons...
1 Didn't want to have to find a place to fix flat right away not knowing where a flat might occur and how far to tire shop.
2 if tire is destroyed, Sailuns are not found everywhere (yet) so didn't want to have to buy some other tire to get by and then have to get another Sailun so they are all the same again.
3 for $125 (when I bought mine) it was worth doing for just plain peace of mind. Sold all 7 "old" tires on Craigs List. win-win.
My guess is the spare will never need to hit the ground cuz the Sailuns are awesome tires! But I have another if ever needed.
2015 Grand Design Momentum 385TH
2015 Ford F-350 DRW 4x4 Lariat