[evincarofautumn]

I think it is worthwhile. If you know a less mainstream language, you can find opportunities to use it at work when it provides a significant advantage, such as being more naturally suited to a particular problem, or having libraries that do what you need. You can also borrow useful ideas and patterns from it and bring them over to the language(s) you do use at work, if they offer ways to improve expressiveness, correctness, maintainability, and so on. Knowing an array of significantly different languages gives you a far broader array of mental tools to use in the design and implementation of software systems than if you stick to one language family or paradigm, even if you only ever use a few mainstream languages at work in practice.

For example, I didn’t have any idea how to do parallelism and concurrency correctly in C++ until using them in Haskell and porting the ideas back over. Same goes for a lot of techniques that were easier to learn in other languages and translate over, like writing parsers, implementing constraint solvers, enforcing complex invariants with types, &c.