I am using Shiro together with Vaadin, but the following should be true for all GWT based rich clients.
If you are using these kind of frameworks, you mostly want to handle login and logout within the application itself. If this is the case, you absolutely don't want that a logout destroys the http session. Because that will alert the user that the session is gone and will force the user to reload the whole application and starting from scratch.
Please: Just give me the possibility to do a user logout without ruining the http session. As a workaround I inherited from DefaultWebSecurityManager and overwrote the logout method to do everything but the http session invalidation. But that's a lot of cut and paste code and it could be easily provided by introducing a configurable setting or parameter for the logout.