If the JavaBeans in the session are not serializable, the JVM will not know how to marshal the data across a network, and unmarshal the data on the other side. As a result, your applications will fail.
In a distributed environment, the data you put into your session must be serializable, otherwise you will face serious problems when you move to a clustered environment.
<opinion> Making a bean serializable is not a dificult thing to do. Any programmer too lazy to conform to this interface when developing code deserves whatever runtime errors they run into at deployment time. </opinion>