From Naoki Yoshida's response in the Live Letter, I believe this is only a temporary restriction, until they see how the RMT are going to try to operate in ARR.

As another poster mentioned, this restriction allows for more granular tracking of the exchange of goods and gil via mail. True, it does nothing to prevent RMT from accumulating wealth and goods. It just forces them to essentially limit the exchange of those goods to face-to-face transactions.

In the meantime, your primary character and your mules could "theoretically" join the same Free Company and use the common storage that is provided by that to help facilitate the consolidation and control of your personal inventory.