I wonder why most re-breathers only have 3 cells? Cost?
Mine runs 5 cells, 3 into the controller, 2 into a Nerd, that's true redundancy. I personally don't like the 3 cell only rigs (not the rigs themselves as such, but the logic of only 3 cells and piggybacking off two of them for the backup). With a 5 cell unit, You can lose 2 cells and its still a div-able rig.
I also like split canisters so even if one breaks through the sorb due to poor packing, the other should pick it up so minimising the risk of a CO2 hit. Maybe its just me but I do like redundancy, particularly with re-breathers. I can lose the controller and still stay on the loop by driving it manually and using the Nerd. I can lose the solenoid and stay on the loop and drive it manually. Controller battery goes flat, drive it manually.
The only thing not truly backed up is operator error (although if its in auto it should try and keep you alive, which is a good thing but it wont save one from stupidity)
Man, you'll completely hate the units that only have 2 cells then.