From Developing and Testing Message-driven Bean Applications with the MQ Simulator for Java Developers in WebSphere Studio Application Developer Version 5.0:

New to Application Developer Version 5.0 is a very productive and lightweight environment that enables quick testing and debugging of MDBs and other JMS applications in a unit test environment.

So far I think it's the first time I see the adjective 'lightweight' applied to WSAD. And productive... when you are able to code and stop to fiddle with the embedded server configuration, it's perhaps true.