I've read that argument before, but IMO any issues that would cause an a/f variance from one bank to another is something I would notice without an a/f gauge. Unless a jet gets plugged or something it would have to be a mechanical failure of some kind to make one side get out of whack with another. I could see where having one o2 sensor per cylinder could be advantageous if you were hardcore tuning, but I'm skeptical as to the idea that one o2 sensor per bank would supply you with any new information over just a single o2 sensor.