I agree with you. It's a great exercise for teaching body awareness, but I never do it when I'm receiving technique, and I never teach it as anything other than an exercise. Quite apart from issues of leaving yourself exposed, etc., I feel like it is very difficult to do a backwards roll if you are actually thrown. It requires a certain amount of pre-emption from uke, which hinders nage's learning.
IME, a back roll is impossible if you're thrown for real, but never say never, I guess.
I do think a back roll makes lots of sense in non-budo applications, but I haven't figured out the budo application for it yet although I've played around with it.. one can bail out of a technique early and do.a back roll and be ready for an incoming attack, but it's rather like a spinning strike.. all about timing and if your partner isn't prepared to handle it. If they are prepared, giving up your back is the worst thing you can do.