Armoire is basically only for storing items that the game is able to confirm you must have by other means, mostly achievements. In a sense, it's not even actually storing the items, it's just an easy access to discard and reobtain those items at no cost. Armoire only exists for the items that essentially will carry very little data when "stored."

As for the idea of "storing without taking up a slot," it's tantamount to expecting them to add a whole new layer of coding to everything just to make that work. Items can't just "not take up a slot." Every item carries a set amount of data with it, and the slot limitation is in place because of that. It's effectively the same arguments as the classic Fisher Tackle box debate. Why create a special set of rules and inventories when they could instead work on increasing inventories all around?