POJO makes business logic reusable across frameworks. For example it will possible to move a domain object from Hibernate to EJB 3.0. I am assuming that external metadata file is used.Using Annontations would mean still changes are required.
Currently EJB business objects are tightly coupled to EJB framework as they implement Entity, Session interface. Ofcourse you could move the business logic to a Java class and call the java class from the EJB bean.