Earlier quoted context omitted.
I'd say, more than anything, the benefit it provides is more personnel-related than anything. While there are definitely dedicated server-side and client side JS people, speaking the "same language" helps within teams and allows for some shared responsibilities. That's appealing to large organizations with decent turnover.
I think you've touched upon the strongest argument for using JavaScript within a server stack. That said, that argument seems less strong for companies the size of Netflix, Yahoo, and so on. They don't exactly have problems recruiting top engineers. Curious stuff.
It also encourages an ecosystem wherein almost any developer out there can work on either side for you.