What, again, is the case?
I agree that there are many good reasons for using scripting langauges within the Java environment. Your Groovy example hints at some of these reasons. I'd point out the superior enterprise resource management of JRuby over standard (C) Ruby as another.
The bulk of your article, however, seems to make the case that one should use scripting languages in Java to keep things "pure Java." Only Java-centric programmers can really take stock in this argument. Programmers who are language agnostic will not understand the benefits of JRuby, Rhino, Groovy, etc. from your arguments.
I believe there is an excellent case to be made for using the JVM in this way, and I'd like to hear it from someone with your experience.