Is a me.com address only used for email in iCloud (outside of brand new accounts)?
Yes, it is only used for email. My Apple ID is an old .mac username which wasn't associated with a MobileMe account. It allowed me to set up iCloud using only that account name. When I turned on iCloud email, it prompted me to set up a new @me.com email address, which I did. But I still use my .mac username to log into icloud.com and access the iTunes store. The new email is only used to send and receive emails.