I don't know how anyone can say it was unnecessary, or that any of it was out of nowhere. All development with Eleven happens in this episode and it makes complete sense. It's not a backdoor pilot, but set-up for another storyline for next season (of which I believe the Duffers intend to be the last, based on their Nerdist interview).
It's Eleven finding her purpose and resolve in a powerful way. I really enjoyed it. It was a break, sure, but a break that worked. To emotionally prepare us for the grand finale.
I loved it.