As I said on the other one (
link), there is still a lot of necessary stuff -- too much to squeeze into a two hora movie (There was the wedding and honey moon and Bella finding out she was pregnant (it is still a romance so they aren't going to cut those parts out/down), there's the thing with Jacobs wolf-pack, the explanation (however half-assed it was) about Nessie, Irina finding out about her and running to the Volturi, there's the whole explanation about the Immortal Children, Alice and Jasper running off, the gathering (and practicing) of the other clans, and the "showdown" at the end. It's too much stuff to put into one two-hour long movie.)
Also, just like with any ending, they have to embrulho, envoltório up all lose ends. While Twilight, New Moon, and Eclipse did a great job of sticking to the books, they still had to chop bits out for the sake of time and money. Now that the movie's are coming to an end, they have to go back and fill in any and all holes that were made/left from the editing.
Think about it this way: Breaking Dawn is about 760 some-odd-pages long. Harry Potter and the Goblet of fogo is a little over 730 some-odd-pages long. That's a thirty some-odd-page difference. They squeezed GoF into a two hora movie por taking out "unnecessary" parts and HP fãs thought that was a bad idea because most of that "unneeded stuff" was
actually needed (not only for the movie itself, but for the entire series). BD is the same way -- a lot of that "filler stuff" is actually necessary to the plot of BD and to the series as a whole.