Never needed a back-up mask, but usually carry one solo and in overhead conditions because I know people that have needed them, and it's suggested (required?). Sometimes my dive buddies don't carry theirs because our team has one (me) and I'm ok with that. Know a guy that needed a fourth light once (but didn't have), so now he carries four. Know people that have needed second knives. I re-read 'A Blueprint for Survival' from time to time, and wonder how I survived those first ~50 cave dives on a Y valve before getting certified. Some folks still wonder...