I never understood the one account to rule them all mindset. It drives me nuts when my wife uses the same password for every service she ever signs up for, and it seems that using the same account for everything is only mildly better (reduces the recovery time if your password is compromised to updating a single account instead of dozens).
Thank goodness for KeePass and random password generators!
Adding Facebook as your sign-on adds another significant point of failure.
I realize that by the nature of the problem, you can only fulfill half of it. I also know that it's probably a pipe dream since lock-in is so valuable, but that is what I would really like to see from my music services.