The interface is not strictly required under EJB3 and your question has nothing to do with Webserices per-se.
The reason for a Remote Interface is just that - Remote (or client). If you don't distribute a remote interface, then you have to distribute your EJB's. Deploying server side logic is a no-no in my book.
So, to sum up, if you expect a remote client to use RMI to call your EJB's, then create and distribute a remote interface.
...Deploying server side logic is a no-no in my book..
Should have read:
Deploying Service side logic to the Client!