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.
I've done pretty extensive development in Ruby and Groovy (as well as a little Jython using Grinder as a load testing tool).
Of the 3, I prefer Groovy the most. I like the closure syntax the best and I think that it has the tightest integration in how it works with other java classes on the JVM. It's been a little while since I last used JRuby, but importing Java classes and working with the classloader in JRuby didn't feel as clean to me.
The fact that Groovy is also essentially a superset of Java means that the huge population of Java programmers out there will have a quicker uptake time in picking Groovy up over Ruby/JRuby. They can start programming it like it's Java and slowly start inserting idomatic groovy as they pick it up.
More to the point of what you're asking, I think that another advantage of Groovy is that the language that you go to when you want to optimize something is almost the exact same syntax, it's Java. If you're working in the Ruby or Python worlds, you're going to have to go to either C which is a big shift or Java, which is also quite different than those languages. Programming in Groovy tends to help keep your Java skills somewhat sharp as well.
If you have particular access to a Ruby or Python infrastructure, or a team that has familiarity with those kind of environments, then I could see choosing one of those other languages.
Really, all 3 of them are very nice languages and what you pick should depend more on the problem that you're trying to fix and the resources that you have available to you. Once you've become proficient in one dynamic language, picking up a second or a third is much easier.