But, but, but that 'backplate' helps a lot when you're running away looking for coverBy the time you go through that maneuver, your shot. Not buying it.
The only thing I think might be good with a backpack is that almost everyone I've seen has a pocket in the back. Perfect place for a plate that one could hold in front of the vital areas in a situation where you have time to get it off your back and hold it in front of you.