I've never liked the phrase, I think this pretty much nails why:

Originally Posted By: ireckon
The beginning part of the phrase is too much hyperbole, specifically, "one is none." If that's true, what is "none?" For example, if I have one good fire starter in my bag, is that practically the same as having zero good fire starters in my bag? No, that's ridiculous. The comparison is not even close.

I also find this to be true:

Originally Posted By: hikermor
I have usually seen the "one is none" routine applied to guns and knives, where it is just plain silly, seemingly an excuse to sell one more firearm and one more blade.

While having a backup blade is fine (and I do!), I've come to the realization that the probability of needing it is extremely low. You have to compound the probability of being in a true survival situation with the probability of either losing or breaking your primary knife (the latter being near impossible with many quality fixed blades now a days). This is the math that is important and, I have a feeling if one was to crunch the numbers, the probability of that backup blade being critical to survival has got to be getting close to lottery winning odds.

So, while I have no issue with having a backup blade, I can't buy into the idea that to not have one is some disastrous failure of planning.
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Victory awaits him who has everything in order — luck, people call it. Defeat is certain for him who has neglected to take the necessary precautions in time; this is called bad luck. Roald Amundsen