it's a valid point you raise. I once drove a couple hundred miles with a flat rear dual until i was able to get to a big truck tire place. That would not have been possible with a super single....(heaven forbid i actually have to stop and change my own tire)
the other side of the argument is that running a super single in the rear is really no more dangerous than a steer tire. If you have a steer tire blow out the consequences seem worse than having even a single rear tire do the same. Another thing is that bus tires really don't go flat very often. I've ran tens of thousands of miles on rock hard baldies (on the rear) that were no-where near dot approved and in all those miles only had 2 flat tires that i can remember.
Quis custodiet ipsos custodes (who will watch the watchmen?)