The problem is really that parties has email, where at the least it should also be in users. You can have it as null as it is now, though that isn't a great solution either. Then you can send email to a user as opposed to a party (assuming you also keep the one in parties).
By having email in parties, you have implied that a party is an entity with email (what about groups which use parties for their relations?)
This really isn't a difficult patch. I already have it. The biggest problem is making sure that existing code that may depend on this faulty assumption about parties having email don't break.
For the record, I don't volunteer to do this, but I at one point had a site that required anon logins and optionally set an email if the user wanted it. That is where this came from, multiple entities with the same email are a similar datamodel issue, but not the motivation for my patch.
--Dave,
RE: sending email. If the model was changed and in addition to email you could use handle as a login, if the people don't put in the right email, what can you do. This is still an issue today. Also, if it is null, you can't login, so you would have to hack the pages to allow handle as an auth type. You can fairly easily stop multiple emails from going out, but unless you set verify email, I can make up email logins all day. How many mailto:bill@microsoft.com or similar do you have in your sites.
That is the real reason that most sites now do not require email
