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?
