I'm just curious what circumstances people envision that will force them to use their backups. Some things are obvious, matches and bic lighters can get wet, compasses can break, but what about knives? Are you worried you'll break your knife, that it'll fall out of the sheath, that the sheath will break, that you'll just forget it somewhere? I've seen some kits with 3 or more knives, and I just see the point in having more than 2.
I mean, I carry a SAK in addition to my belt knife, but mainly for the thin blade for cleaning fish and the saw, I'm probably more likely to lose or break the SAK anway. But other important stuff (e.g. water purification tabs, firestarter) I just keep in a pouch on my belt so I won't be separated from it.