It will not accept a new contact without an email, even if they are not subscribed to any list. Therefore, we are forced to enter a made-up email just to save the contact. But if say for example we use firstname.lastname@example.org, it will "update" the last contact that was entered with that email with the new contact's information. Which means we just lost a contact. Not only that, we track a lot of our customers by ZIP CODE, and never email those particular contacts, so in certain circumstances, an email is unnecessary requirement.