The LOTR books aren’t much longer, and Jackson had to heavily rewrite them to get a comprehensible and filmable movie. Scenes that take a few paragraphs can require tens of minutes of footage to accomplish. The announcement did say (and the trailers apparently hint that) Gandalf would be seen going to Dol Guldur, at least.
This way, we’re more likely to get a good version of the battle in the caves, the arrival at Beorn’s, the battle with the spiders, and the party’s imprisonment in the wood-elves’ palace. Hopefully, we’ll also get a good build-up to the opening of the secret door, as well as the battle on the lake and the Battle of Five Armies.