The three-sectioned staff has a wildly different use from a naginata. The former is a highly unpredictable weapon, hard to defend against thanks to it's ability to strike in an arc, the latter especially deadly against mounted and larger opponents. The bo-staff is, in and by itself, one of the most versatile defensive weapons out there. The scythe... well, would totally be a non-issue because the naginata is superior to the scythe in about ever aspect outside of maybe a cool- or farming factor. However putting the other three together is a pretty big deal. You could even transform it into a makeshift kusarigama the way it's set up; adding yet another function to the equation (though I don't think the chain would stretch that far.)

The thing is that the way the Library works, it's safe to assume that every weapon is an epitome of craftsmanship which cannot break or dull unless specified otherwise. I mean, even bows don't run out of arrows. If your weapon doesn't break or succumb under force, and for some reason has perfect balance in spite of only one end having a blade, you're cancelling out all problems a real world counter-part might have, leaving you with, indeed, multiple weapons in one slot.

EDIT: Also to give you another example, we had two guys using cannons as weapons, but they took side-effects like having cooldowns or required to be mounted for a post to balance out the power, even though they could probably carry it around with ease thanks to their POW score.