Going back to an earlier discussion for a moment.
If Jackson, Walsh, and Boyens were committed to telling a faithful adaptation of The Hobbit, both literally and spiritually, then the Necromancer subplot is wholly unnecessary. And I say that as someone who loves Gandalf.
The Hobbit is Bilbo's story. The narrative is structured around this tiny, seemingly insignificant person being swept up from the comfort of his ordinary life and into journeys both physical and emotional. That's ultimately what helps The Hobbit achieve its sense of wonder and adventure. We, the readers, are witnessing these fantastic events through the eyes of a character who also is experiencing and seeing these things for the first time.
You don't need to worry about developing each and every dwarf when you frame the story around Bilbo. We should see the dwarves the same way that Bilbo does. At first, a loud, overwhelming group where you struggle to remember who's who. Further driving home that initial feeling in the narrative that Bilbo is in way over his head, out of his elements, and flatly the odd man out. It makes you uncomfortable.
And when Bilbo later assumes more command and authority within the group, again, the dwarves themselves aren't the focus (especially on an individual level). It's the fact that this little hobbit, initially overwhelmed by everything, has the respect and attention of this large group. Following the fight with the spiders, these thirteen dwarves, as a collective, are immediatey looking to him for suggestions and guidance. That's what's important. And the main dwarf characters that do get more to do in the book (Thorin, Balin, Gloin) are developed through their specific interactions and relationships with Bilbo.
Now, enter Gandalf.
You can call it whatever you want. The "mentor," the "guide," the "safety net," the "plot device." That is Gandalf's purpose in the book. And just as with anything else in the book, we see Gandalf from Bilbo's point-of-view. We see this mysterious wizard who comes and goes as he pleases. We see a character who has other businesses and workings in Middle-earth that we aren't privy, because we aren't intended to be. They are bigger than Bilbo, they are bigger than the dwarves, they are bigger than the quest, they are bigger than us. And all the while you have Bilbo and the dwarves asking (repetitively):
"Where's Gandalf?"
"How could he possibly leave us like this?"
We as readers are asking these questions too. And that's what makes Gandalf that much more compelling. Frankly, Gandalf is the most wizard-y in The Hobbit, in part because of the mysterious nature of his role and now he steps out of the narrative without an explanation besides "I have other matters to attend to." You show what Gandalf is off doing, and you lose that mysterious, almost omnipotent quality. In a weird way, taking the time to break away from the main narrative to show what he's up to almost normalizes him. And again, it detracts from Bilbo's story.
Then, of course, there is the simple fact that not once does the Necromancer or Dol Guldur or The White Council come up in LotR. Not a single mention. There is no necessity for it in the cinematic narrative. And really, when you've got Gandalf and the White Council combatting Sauron and the Ring Wraiths (the main antagonists of the following trilogy) and basically making a play that could have an impact on the bigger-picture of Middle-earth as a whole, how is Bilbo's glorified treasure hunt with thirteen dwarves supposed to compete with that. In my opinion, all Gandalf's subplot accomplishes is upstaging and undermining the main narrative.
