LinkedIn isn't constrained to the JVM, as they had some apps running on CRuby 1.8 and node.js, but they do heavily favor the JVM. JVM interop DOES help with much of their existing codebase which is Java heavy.
Yeah I agree to interoperability. Makes it much simpler to have the same Java stack across the distribution. Either way, that's a big boost to Play framework!
It's been a while, but there was a lot of use of Groovy on Grails a few years ago for exactly this reason. I was a strong advocate of JRuby when we did run Ruby code, but it didn't get very far.