A back up is fine, but what are you going to do if the computer packs up? Head up and call the dive? If so the backup wasn't much use was it?
I wouldn't bother myself.
If you want a belt and braces approach a
second computer is the answer then if one goes wrong (about as likely as seeing a mermaid unless you buy an Apeks Quantum
) the backup can then complete the dive and indeed go on to do the second dive of the day and the rest of the week on a liveaboard/weekend on a boat. The former of which is about the only time I would bother. The cost of a backup 'puter compared to losing a days diving on a livaboard makes it worthwhile. (hey go rent one!!)
Redundancy
is a good thing, but hardly a priority on shallow recreational dives. A depth gauge without a timer and tables doesn't tell you much you can't guess just by looking up at the surface.
Chris