Why learn anything other than x86 machine code? ;)
Seriously, from a software engineering perspective, the past decade has been a peculiar time: JS used to seem like a joke to many, but it's got past that and is now a serious contender as a platform compared to established ecosystems. However, we're also starting to see other languages include JS as a target platform; once that matures, those other ecosystems can piggyback on JS's features, and it loses much of its appeal.
From a computer science/hobby programming perspective, JS is a 'Jack of all trades, master of none'. If you only know JS, you might get really good at churning out code, debugging language-specific nuances, sculpting libraries to your whim, etc. but I would argue that you won't be really good at the big-picture stuff.
Without the perspective of multiple paradigms, you might not see the wood for the trees. Your thinking will be a fuzzy mixture of important ideas lost in a sea of JS implementation details; you won't have the clarity that comes from thinking about a problem from multiple perspectives.
It can be difficult to learn a different technology; especially if it's not by your own choice. The key is that, once you've got past the initial "It can't even do X, Y and Z!" stage, you'll realise that:
- It *can* do X, just in a way you didn't recognise because you didn't fully understand X
- It can do A, B and C that you never even knew existed
- Y is actually an anti-feature, which you didn't realise the problems of before
With all that said, if you're trying to learn new things, there's not much 'bang for your buck' going from JS/Node to Python/Django, Ruby/Rails, PHP/Symfony, or even whatever Perl's up to these days, since those languages are all very similar; of course you'll find a few nifty frameworks/approaches which are popular in each, but if you look around on GitHub you'll usually find slightly-crappier reimplementations in most of those other languages.
If you want to get some real "Aha!" moments, try something out of your comfort zone like Erlang, Haskell, C, FORTH, Prolog, Maude, Scheme, etc. (admitedly JS is like a cheap knock-off of Scheme, but those missing pieces like call-cc, tail-recursion and macros can drastically change the mindset of its users).
If you want to get stuff done, stick to what you know ;)