Earlier quoted context omitted.
"the annoyance that is "Sign up with a username, email address, and password"." I know of an easy way to remove 33% of that ... Email addresses are globally unique, and everyone already knows theirs ... email + password seems like a simple, well accepted and workable solution. Right ?
Emails may be globally unique but they aren't static over time. I have some accounts on websites where the login is an email address I no longer have access to.
My appleId has an @mac.com email address which means it is for some reason immutable - I can not change my address. Jobs only knows where emails from apple to me end up, I really don't do much with my appleid anymore other than re-authorize the appstore every month when I need to update xcode.