I like the ability to recover and get back up into nage if the shihonage fails. I can't see how you can recover when falling straight backwards. I might be missing something though?
If you stay on the balls of your feet instead of being flatt footed and you really push your center up, you can come back up easier. Sensei likes to use me to demonstrate and I get put in that situation so he can demonstrate why you should stay on the balls of your feet (for the sole reason you mentioned- you can come back up or walk around backwards if nage decides to walk you around)