This is true. Though getting an iMessage from john.doe@gmail.com who isn't in your contacts isn't as uninformative as getting an iMessage from +1-234-567-8900.
If you're iMessaging them, they either know that 1-234-567-8900 is John Doe, or chances are good that the content of the message is "Hey, this is John Doe, got a new number."
Touché. Though I suspect it might be prudent to further explain that that means they need to add your email address to your contact, as their mind will be blown at that point.
