Why do people want a mailbox specifically for the purpose of trading items? Why not just expand retainer functionality?

1) Allow any character on your account to summon any retainer.

2) Allow your retainer to have a 'Delivery' area. You add an item (up to 10, like the other sections; no gil) to this section and then type the name of the character you want it delivered to. Then, that character can summon this retainer like they would their own, except that they can only see/retrieve/add items to the delivery area.

I've always been of the opinion that mailboxes should be for correspondence in games, no item delivery - there are more effective ways of handling the latter than the silly FedEx system we see in many MMORPGs.