Reason: To make it compatible with other plugins that rely on wp user data.
Current obstacles:
there two different data sets for customer name and email (Amelia und WP). When profile gets frontend changed by customer, the customer must for example change his email address in 2 different profiles, which is confusing, and misdirected mails are predictable.
in some cases already logged in user have to log in or fill in name/email address again to book with Amelia. The logout and login in of Amelia is not a fully implemented logout and login of a wp user (as much as I have seen, depending on the Amelia settings, of course).