Account security should be a top priority.
Using email addresses for your Account Names (Login Names) is a security risk and makes it much easier for accounts to be compromised.
Account names (Login names) should be unique not email addresses.
If an account is hacked (compromised) or if there’s a security risk, players should be able to easily change not only their password but their login name as well. This would essentially make account security much better.
Lets face it, it’s much easier to get a person’s email address than it is to get their login names and passwords, especially since most people only use a couple of email addresses if that.