Since we have it for wardrobe and collections, this would be handy c:

It may be handy, but it's not very often that I'm out there specifically buying a weapon.

Unlike other direct to collections where you can only have one of anything inside it, you can have multiple-same-items in your Armory. (I do this to have different crystals chocked in, and while many are limited not all weapons are one-per-battle-set)