Learning new languages considered harmful? No.

Good post. I agree with almost everything he says.

> For example, Scala is great, but I don’t use it much because there’re not enough cases where its features are required

IMO with languages like Scala, JRuby, Clojure etc. that are built on a stable platform, its easy to switch to them. You don't necessarily need to use every feature, even a subset will make you more productive. The use case would be anywhere you use Java.

> I know Python, Ruby, Scala, and Groovy (...) but when I need to actually get things done, I still get them done more efficiently in Java, as long as "more efficiently" doesn’t mean lines of code.

I totally agree with this. Recently I picked up Ruby and really like the language and would love to use it. However, I am very comfortable with Python (and its libs, docs, django etc.) and am not really as productive with Ruby. If I would have picked up Ruby first it would have probably been the other way round.