As a rule, I'd want to know one client-side language and one server-side. With Node, JavaScript blurs those lines. But as others have said, it depends entirely what you're wanting to do.
Pretty much anything web-based, you won't go wrong knowing JavaScript. And even knowing a little will mean that you've a better understanding of what those jQuery libraries you're importing all over the show are doing for you - and how you can make them work more to your advantage.
Away from the web/desktop/etc, knowing JavaScript would help you with Node-based things, Tessel, that sort of lovely kit. But not a lot more that I can think of right now.
The big advantage of knowing any programming language is that it gives you a better idea of what most other languages are up to when you sit down and try to learn them. You're already thinking like a programmer. Unless it's WhiteSpace or some similar esoteric wonder, in which case all bets are off.
So, must-learn? No. Useful? Definitely. No knowledge is ever wasted.