Also this!!!Except that it doesn't. Friending your alts would have two effects: you'd be able to share a house, and you'd be able to send stuff to them through mogmail. Sharing a house would clearly help SE who has some difficulty in expanding housing enough for everyone. Sending items through mail doesn't change anything about retainers, since we can already pass stuff through the FC chest, or sell to ourselves through the MB.
People who claim it would affect retainers seem to think it would be used for mule alts, but all it does is take what's currently possible but awkward and make it still possible but awkward. There's NO CHANGE in that. People want mail access to pass items to the character who would use the item, not to shuffle them off to where they aren't as accessible.
Oh, plus there's the fact that this was an issue before paid retainers even existed. That's proof enough that the rule isn't about them...
SE has already told us that the reason we can only send mail to friends is to cut down on RMT. I certainly wouldn't want every spambot to be able to flood my mailbox with their garbage, so to that extent it's a good thing. But the rule simply doesn't make sense for the specific case of alts on our own account. Nobody's going to be spamming themselves or buying gil from themselves. It's perfectly reasonable to make it an exception to the rule since it's already an exception to the reason for the rule to exist.