Flashbacks of Gandalf in Dol Guldur happen in the book (finding Thrain and the key) so they could expand on that. Matching the tone to LOTR is nothing new either, see "The Quest of Erebor". But do we really need to see the White Council fighting Sauron?
We all know the business with Gandalf coming and going from time to time is originally just a plot device, as well as alluding to myths and legends about sages and guides, like Merlin for instance. The Necromancer, the White Council and Dol Guldur are tangential or peripheral to the actual plot of The Hobbit even if we know their true significance. Our inner geek might rejoice to see them in more detail, but is such added detail really needed in what is still titled "The Hobbit"?
Let me take Tolkien out of context:
Though I am most peeved and saddened by Frodo and Narsil. Again, Frodo has no place in The Hobbit and is only there because LOTR was filmed first. Narsil I'd have saved for the movie itself as a subtle Easter Egg. When it's given the spotlight in the trailer, it's not an Easter Egg (because you have to look for those). It feels gratuitous.We must be satisfied with the soup that is set before us, and not desire to see the bones of the ox out of which it has been boiled.
Thing is, the trailer got so much right or acceptable, but the parts it doesn't get sour the whole thing for me. Just as the two finished movies probably will, and just as the trilogy has.
(originally from TORC.)