"... an email address is guaranteed to be globally unique ..."

While the statement is true, the accompanying implication that this provides a globally unique identifier for a person is not.

There are many families who share a home PC and ISP account - multiple people, one email address. While they may share a home, a PC and an email address, they probably don't share a wallet and certainly don't share medical histories.

On the other side of the coin, many people have multiple email addresses - a home address, a work address, one or more web-based mail addresses.

Microsoft have already solved this problem though - lets just assign everyone a GUID :-)