As for the logic of using a single language throughout your stack, I would argue two points:
[1] It's a myth. If you are a professional software developer, you are going to learn multiple languages... and if you're a GOOD software developer, then this excites you rather than bums you out.
So you're using JavaScript on both the client-side and server-side. Okay, how do you get information from your database without learning SQL? Just stick with MongoDB as the only datastore you will ever use? Document stores are great for the things that document stores are great for (another tautology), but suck horribly when you try to shoehorn them into situations calling for a relational database.
What about the HTML and CSS knowledge you already need to work on the client-side? Do those "not count", because a JavaScript developer usually happens to know them? Nonsense. Doing real work requires knowing multiple languages, because different ones are better suited for different domains.
[2] I would argue that the skills and considerations required to do non-trivial work on the server-side are generally different from those skills needed to do non-trivial work on the client-side. You may be smart and/or experienced enough to master both domains, but you approach design and coding issues differently in those domains.
If you take a developer who is senior on the server-side, but is so junior level on the client-side that he or she barely understands the DOM, then they will struggle at first in the other context. Vice-versa if you take a senior client-side web developer and throw them into the backend world. You're going to need to learn completely different skills, quickly. I'm not convinced that clinging to your "home" language really speeds up that process, rather than impairing it.