You'd only think that if you'd had no experience with databases, no offence.
I can certainly see why tracking multiple email addresses for an account would cause problems. Rather than storing the email address, it would need a table of email addresses, that's another link, and the whole infrastructure would have to be changed to allow for this extra data. By using a single email address you have decent security, and a decent control over the user for tracking and authorisation. Changing this address is one thing, but changing the whole system to allow multiple addresses is the sort of thing making database coders bald. Seems like such a little thing, but it's really a can of wriggly SQL worms that'll take your hand off as soon as look at you
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