I've only needed one when I used scubapro regs.I use Atomic regulator, I don't need a backup regulator for myself at all. Never needed one, not even once, in all of my extensive travels around the world in the past at all. When I carry one it is when I am leading a group and I may need one for one of my fellow travelers. I do carry backup computer(s) however
Since I switched to Deep 6, I haven't needed them either. But I always carry two regs as I like to sling an AL80