I dont see how a backup can be a failure point. A failure point is a place for one item to fail, such as an oring sealed port. Having a second computer doesnt make the first one more likely to fail on its own accord.
Not all diving can be terminated immediately when a computer fails. Even if it can be, many operations on trips wont let you back in the water for 24 hours unless you have been using tables the entire trip, otherwise you might be guessing at your nitrogen levels. Using your buddies computer is not a good option at all. And, tables only work if you have something to keep track of time. IE, a backup dive computer or bottom timer J
Backups can be kept out of sight, but I always worry if its still there, or still working. If either of my computers fail, Im turning the dive. I prefer to keep everything on my arms, using DSS bungee mounts.
They have a few different sizes for different computers, hopefully one fits yours. You could always tie a boltsnap to it to store it like the Oceanic BUD, if it fits tightly then it should retain the computer inside the rubber boot properly, but I still think using it on your arms is the best option.