Thinking thinking thinking...
Gandalf going off in The Hobbit was really just an excuse to give Bilbo the oportunity to 'grow' as a character (and hero, even). Gandalf went off to chase the Necromancer away from Dol Guldur (I'm not even sure it was to chase Sauron away to Mordor at that stage of writing). This is an excellent reason why it should not be part of the movie - it was never, strictly speaking, part of the book.
Bilbo is the Story. The only diversion should be the slaying of Smaug, as this was IN THE BOOK. And should be shown after the Party have had time to begin to wonder about things - perhaps in Flashback. All that wandering around inside the Lonely Mountain with them wondering where Smaug had got to was truly suspenseful. Let this play out - and then do the Flashback.
Hey, how's this: don't fix it if it's not broke!
Odo
