I only use my computer, but I do have 2 of them at all times so I still have redundancy. As you're probably noticing, how much redundancy a given diver wants changes from person to person. On liveaboards, technical dives, etc, you'll find people tend to prefer more redundancy in things that could either impact their ability to keep diving (not knowing tissue loading would make doing the 5th dive of the day irresponsible so a backup computer is preferred by many for instance) or their ability to safely complete the dive.