Backward rolls

So far all the arguments for backward rolls have been based around getting up, and creating distance. I'd say that is a valid point, provided you have the time and space to use it. However, if you are being splatted at nage's feet, then I don't see it as a realistic option. The way we do tenchi-nage for instance, there is not a lot of outward projection, and it is all straight down. At speed, you even have to do a baseball slide type of ukemi. In that sort of situation, I don't see a backward roll as realistic.

I am still curious - since the back role is ubiquitous. Where does it originate.

Shodokan doesn't do them but that might be due to technical reasons rather than it did not exist when Tomiki was a student.

Well, I would qualify that statement (ubiquitous) with saying that it is a part of our warmups, but again, I can't think of any techniques/waza/whathaveyou that have a full backwards roll as the proper ukemi. FWIW my take on it (totally without any means of validating or confirming) is that training in a full backwards roll was about simply teaching body skills. The ability to fall backwards confidently and go over if need be to regather balance. So I've always considered more a training thing than an actual "ukemi" you would likely take. Interesting to think about it more deeply, however...