The Language, (in this case JavaScript) is probably the smallest part of domain knowledge needed to be a good front-end or back-end engineer. Front-end engineer, know JavaScript? OK, optimize these queries, do we need to cache, make temp tables, map/redux against the DW, etc? Hey backend developer, know JavaScript? Why won't this render the same in Chrome 33.1 as it does IE 10...etc...
JavaScript is a convenience language across the stack for when the team is small, or even a single person.
The problem with web dev (I've thought about this a lot) is really that the browser is kindof the hinge. So no matter what you do for unifying languages, even porting modules so they're same on node front to back-end... you still can't really ever get to a point where you're unit testing pretty reliably through the full-stack.
So might as well test parts in isolation & who cares if there is one language throughout the whole stack as long as your devs are good at the languages required.
One day I think the impedance between server & client will disappear, but only when the browser's role as a rendering engine diminishes or finds better integration/intimacy with the codebase.
(and incidentally, i stopped caring so much about this struggle once i learned how to develop pretty reliable SOA. Haven't totally given up trying to conceptualize the perfect stack tho)
JavaScript is a convenience language across the stack for when the team is small, or even a single person.