[Falaina]

The JVM is a platform, much like x86/linux or PPC/mac. It's always advantageous to have a better understanding of the platform your code will be working on, but it's not strictly required to be an effective programmer.

With that aside, I think it's certainly worthwhile to take the time and get a basic understanding of Java, as well as survey what is available in terms of libraries (both standard and 3rd party libraries). Some of the biggest appeal of these languages comes from the fact that they're able to leverage all this existing Java code.