[Lunaryn]

While I don't know if SE acknowledges or legitimizes this in any way, it is the case that often times siblings in the same household each have their own character on the same account, and the ability for one of them to go through and snatch the other's belongings with complete ease might be somewhat unwelcome. If one wanted to account for this (and keeping in mind that since the only real barrier (password) is on a per-account, not per-character basis, it is only a question of ease and not of basic capacity to transfer inventory) the best way would probably be that each character has to sign up for the warehouse functionality independently to have their stored belongings show up, complete with the capability of withdrawing from it later.

That said, any more of a barrier than a fairly simple sign-up is unwarranted given the understanding of the security of a character being at the account level.

Having some ability to look beyond character into the resources of an account might have some additional utility in terms of error-prevention; it wouldn't be possible to typo and accidentally send your items to another player within this kind of system. I don't know if that's a significant concern, but I have noticed many characters with only slight variations in spelling on FFXI...