Here is my retread that came apart last summer.
, on Flickr
I was about 40 miles north of Mitchell SD, when it came apart. I just drove about 30 mph on it, to get me into town. I knew the tire was no good anyway, just didn't want to to come apart more and tear the bus up. I carry just a spare tire no rim. I probably wouldn't even do that, but the spare didn't cost me anything.
Here is my spare. It's a drive position tire and not a retread.
, on Flickr
Since I was able to drive to the tire shop and having the spare, it cost me $42 to have them swap tires out for me. I did get lucky being the outside dual, saving time.
My opinion on the spare is... If you can get a cheap good spare, and have room to carry one, why not. I would prefer a steer, or all position though. I wouldn't pay good money though to have one with me, and just have it aging without being used. Besides, If I didn't have a spare I would probably just have a tire shop single out a rear long enough to get home or find another tire. Of course you would have to drive accordingly. I plan on getting some weights of the bus this year. I think, even running single rear tire , I would still be within the tires weight rating.