server issues:
If the expense of storage and bandwidth of retrieval is making them so stingy about inventory space, then they should be focusing on more efficient systems.
The armoire is extremely space efficient, because it only stores a single bit (a yes/no of whether we do or do not have the item) for each thing it can hold. A glamour database could do the same. And if (again like the armoire) it were the type of feature where you go somewhere and interact with something to bring it up, then it's not tying up memory space with the "always available" requirements of our regular inventory.
A dedicated crafting mat storage system wouldn't need to remember anything except stack sizes (figuring a couple bytes for the number of NQ and another couple bytes for the number of HQ, it would only take about 7kb per character to store all 1760 crafting mats in the game.) But to obtain that efficiency, it would have to hold all crafting mats. Having a limited number of slots and letting us choose which mats to store there would mean having to store both an item id and the stack size for each slot. That alone would likely triple (or more) the space per item, even without also adding all the additional information that a more generalized item slot (like inventory or retainer inventory) can need.
The same holds true of a tackle box. It would only need a stack size for each type of bait, so could be done much more efficiently than keeping them as individual items. (And again, it could be set up for the list of baits to only remain in memory while we're fishing or interacting with it rather than the always available rules of regular inventory.)
Retainers (or our own inventory) are versatile in being able to hold any type of item, but in exchange for that they're not nearly as efficient.
plenty of space already
That might actually be true for someone who doesn't craft, doesn't collect glamour gear, and doesn't fish. (Or, if they do any of them, does so only to a very limited extent.)
But having to curtail really major sections of the game like that eliminates the justification that extra retainers are a bonus luxury feature that justifies an extra cost. I'm barely into the game's original endgame and still a long way off from its current endgame, and I've already had to resort to getting a third retainer, and will soon have to extend that to a fourth, just to be able to continue utilizing all the game's classes. In a subscription based game, the subscription should give us access to all the game's basic features. This one doesn't.
PS3 limitations
This might affect the UI and how we interact with these systems, but doesn't impact the total amount of information stored, because our client system only needs to hold what we're interacting with at the moment, not everything that exists for our character.
As an example, if dealing with all crafting mats in the game at once would be too difficult to manage (alongside everything else going on) on a PS3's limited memory, then maybe when we go to the crafting mat warehouse (or whatever it is), we ask something like "show me all cloth items I have" or "show me all stone items I have" instead of getting a list of all our crafting mats at once. So long as we can move any of the crafting materials we have in inventory into the system regardless of category (which the client shouldn't have a problem with - it's holding inventory anyway), then having to retrieve mats from storage on a per-category basis wouldn't be too difficult (or no worse than having to both store and retrieve items per retainer).
summary
While we've seen all these reasons cited by people trying to justify the lack of space, none of them really hold up.