Why would I choose to use Groovy when I could use Jython or Jruby? Does the language provide any inherent advantages to make up for the fact that Jython and Jruby skills are applicable to their parent languages outside of the JVM?

Keep in mind that I purposely keeping this question generic, but if there are any advantages that exist in a particular domain, please don't hesitate to describe them.

EDIT
To clarify, If I write some code in Jruby, I can now, in some cases, move that code outside of the JVM if need be, or at the very least I have gained a better understanding of Ruby. Whereas Groovy skills are applicable only when using a language that just exists inside the JVM. Jython and Jruby have this built in advantage, what does Groovy have to make up for this disadvantage?

If Groovy doesn't have any advantages that you've found, and you would suggest just using Jython or Jruby, let me know.

Edit 2
Thanks everyone for all the answers, most of them make the same point, Groovy integrates slightly better with Java then Jython or Jruby.

Follow up
Using Netbeans 6.5 as my IDE I have found that Groovy to integrates better with Java projects then Jruby. I am not sure if lack of integration is a failing of Jruby or Netbeans. But after using it for alittle Groovy definitely seems to have a leg up.