True, there are mailboxes close to most housekeepers in town, but it doesn't help if you are in the middle of a zone and use a Home-Sweet-Home rune to enter your house and then end up needing a mailbox.
And as I mentioned before, using a Convenient Mailbox would work in those situations, but it's one more item to purchase. As would leaving your house and using a purchased Guild Castle Transport Rune (provided you were in a guild that had a castle) and use the mailbox inside.
So I suppose in the end, it's better to have the players spend their diamonds on two or more of the above instead of providing mail service via the housekeeper. Well, it was worth a brief discussion regardless. Thanks for the feedback.