Cable failure is one of the few items that can be hard to prevent or forsee. Even with the loss of any one cable you should still be able to pedal home. Cable life appears to be quite variable, and the actual failure can be in the cable, swedge or gradual stretch over time.
Stocking extra cables is expensive. Besides, cable specs change from time to time so there is no guarantee your old spare will fit on the next generation Drive (We've seen at least 3 sizes of chain/cable so far over the life of the Mirage Drive and two idler cables).
You can best stay ahead of the situation by checking chain and cable tension after each use -- only takes a second.
LocNuts loosening on the current slotted cables are not uncommon (but not easily preventable). If Loctite Blue fails, you can lock them by double nutting -- not convenient (requires 2 wrenches) but effective!