If you plan on storing these beans in session, it's a good idea to make them serializable. Some containers (
Tomcat, that I know of) will serialize all sessions to disk when the app or container is shut down for a re-start.
It then reads them back when it starts up.
If any object bound to session, directly or indirectly, doesn't implement serializable, this whole operation fails.
Also, as Ulf mentioned, in a clustered environment, Tomcat uses serialization for session replication.
I'm sure most containers have similar functionality.