I actually think this is a good idea. An email address is unique and easy to remember for the user.
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.
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.