The stupid stuff is the following:
This was not always required. I think there was a transition period were it was not enforced but only advised on the register page. And also somewhat ambigous like "Enter here your Apple ID username eg. BruceWayne@mac.com" I know two people who entered their (company) name in such a way, but without really having a dotmac address! And then dotmac was changed to Mobile Me and even though me.com was retained for iCloud I couldn't use an "old" Mobile Me email-adress, which was sad because it was a nice address with the initials of my name, but had to make a new me.com iCloud account/address. Really Confusing.
But, really, how would that be different if the user ID was a random name? People could still have "numerous Apple IDs and content bought with all of them".
If it does exist, do you happen to know the URL as I've been looking to do this for a while.
Checking again, that feature does not exist. I was certain I has seen info about doing so, but the official Apple docs say "nope".