1. I can do step-by step debugging from client code to server code and back in a single debugging session (server must be executed in the same process)
2. I can use only one IDE for debugging instead of bunch (Firebug, Eclipse or whatever)
3. Mapping between variables is transparent because I have Java at both sides! I don't have to think which Java type is mapped to which in JSON or in some Webservice
Personally I didn't work with Groovy + Scala + Whatever +Java mix programs but I suppose that similar problems with debugging could appear.
Your opinions, use cases guys?
(I didn't posted it as a comment to Andreas's post because I'd like to stress attention on use cases.)