Absolutely ridiculous comment. You added nothing to the discussion and gave no justification for a frankly ignorant standpoint. I can speak for two of them: JRuby is an absolute godsend even if you aren't massively interested in Ruby - Java libraries/classes + a REPL is a phenomenally powerful combination and makes learning new libraries incredibly easy. Groovy I am not a fan of for significant development, but it's great for DSLs, and in particular gradle is the nicest way I've found to manage dependencies and project setup.
I can't speak to the other languages, but Scala is definitely being used pretty heavily in a number of pretty major back-end systems for stock trading, cloud computing, banking and successful startups like Twitter, Foursquare, Tumblr, Quora etc.
I think by "serious" the GP must have meant literally serious developers, such as those working in huge corporate body shops who are prevented from not being serious by strict regiment. Certainly management in such environs would consider authorization to use any more expressive language than Java as tantamount to handing out dynamite on Halloween.
I can understand that esp. young developers get bored and want to try something new and fancy. OTOH, JRuby and Groovy are, at best, niche languages without traction. The drawbacks of Scala have been discussed widely, also here on hn. Clojure? How many Hickey fans actually write real-world programs?
