You are correct. The email registrants feature is pretty simple and slated to be enhanced in the coming weeks. You can have attachments automatically sent to each person when they register for an event. That feature is already there and has been for a while.
We did note that the attachment was available for the confirmation reply email.
However, in the case were a "mail in payment" is used and that payment must be received before, let's say, a "parking pass" is issued, the follow up email should have the ability to attach a document.
There are many other scenarios where post-confirmation emails could be sent to registered attendees with attachments such as course materials, questionaires, pre-event info, etc.