I think "Single" Registrations should be eliminated. This is because in my opinion you should always have an option for a separate billing contact even if it is the same person as the registrant. Right now the only way to get this is to allow only Group Registrations with a minimum group size of 1. This is perfect, however I would like a completely separate email template for the biller as much of the information I put in for the registrants/attendees I don't want the biller to see in terms of the emails that are delivered.
Dropping individual registrations would be silly of course as most people need it. You don't, but that's no reason to drop the functionality. We never drop existing functionality.
However, you CAN set your event to only accept group registrations. So YOU can drop individual as an option whenever you want!
Rather than making a separate email to configure, it'd probably be better to make some new tags to designate portions of the email as for only billing or for only members. We'll consider this for future development.