Maybe that thing was just taken out it´s original context. It´s just perfect for the military. It´s simple, easy to remember as it rhymes and can be applied to different contexts. You can apply it from everything from having backup for mission critical supplies to taking care of your buddies. I´m not calling military personal silly or dumb but stress and sleep deprivation are sure to take their toll and they are not known to improve mental abilities.
For preparedness context it should be taken with liberal dose of salt. If a backup is sensible take it, otherwise let it be. If you can go to the next shop and just buy a replacement, it may not even be sensible to even own the backup.
For blades having one for rough jobs and one for fine work makes sense. With guns it´s better to have just one that´s kept perfectly clean and in good repair than having two in moderate condition.