From the Cathy's book I understood that statefull session beans are not pooled. However from the Cade book I understand they are!
Could you please point out the pg on Cade book?
Websphere will manage a pool of stateful instances, passivating and then activating them as necessary to serve client requests, in effect, "switching" the client's state in and out of instances as required by requests.
Why passivate? Passivation allow Websphere to manage a small pool of available instances. It can switch the state for many clients in and out of just a few real stateful instances. This is much more efficient use of resources than maintaining an instance for every previous client.