Shore never uses ghostwriters. He hasn't even used an orchestrator for many years. While it's true that many film composers farm out work, most offer an "additional music by" credit now days. Danny Elfman does this. Hans Zimmer, too.
At any rate, this was not at all the case with The Hobbit. "Misty Mountains" was composed by Plan 9 as a piece of diagetic music -- that is, a piece that was performed by characters in the film. (I'm not giving anything away; it was in the 2011 teaser trailer.) It was a good tune -- and fairly malleable -- so Shore wove it into the score here and there ... though, as you've no doubt noticed in the preview tracks, he made it very much his own. There are some thrilling rhythmic and harmonic variations that are unmistakably in Shore's voice.
None of this was done as a time-saving measure by any means. It was a way to create a beautiful, cohesive whole.