Books aren't paced like movies. In THAT case, they probably wanted it to look more visually interesting than Voldemort and Harry standing there talking, flinging a couple spells, and boom, end. It works okay on the page but it's not exactly scintillating cinema. Same thing with leaving IN and actually showing fight scenes in the Twilight movies, while in the books, it's pretty much all "Bella was knocked out, there was a fight, Bella blinked, there was a fight and it was over," etc. Even still it's going to be interesting seeing how they manage to make the end of Breaking Dawn even halfway interesting. With HG they have the problem of getting all the backstory and details that we get from Katniss knowing it into the film without having her do a voiceover. I liked how they used the "TV" format to do it, too, which obviously wouldn't work with every adaptation but fits perfectly in the Hunger Games.