I wouldn't be without a spare tire on anything except my bicycle. Why go through the headache of having to worry about finding a tire while you are on the side of the interstate with a blow out? It'd be a lot cheaper now to buy the tire than to have to pay big bucks for one from a road side tire service.
A blowout can happen at any time for any reason. We didn't have a blowout with our first two RVs over the course of 5 years. We've had one blowout with our Motorhome (just last week) 1 month shy of 7 years. It looks like I ran over something on the road that caused the blow out. I am very careful with monitoring tire pressure and always check the tire temperature whenever I stop. I still had a blowout.
-Michael