Even if you never make use of the new programming language it is almost always worth getting your head around a new way of thinking about problems. You may not ever need to write code in Lisp but understanding what a functional language is and isn't good for is helpful in other languages. If you're building flight control systems then Python might not be the language to do it in but getting deep into it and start understanding why you'd want a metaclass in the first place can help you stricture your code better. There aren't a ton of job openings for Erlang programmers but there are lots for people who understand High Availability and Disaster Recovery and the background knowledge will stand you in good stead.
In short, yes, it's worth learning all the languages your brain can handle. Even if you forget the details the concepts will stick around and help you later.