You need a little bit of action in each Twilight movie (the fight scene in Eclipse was plain awesome)
It would build up more supense. Will the Cullens win or lose? Who will die?
Some of the evil Voltouri need to die (the bad people always have to die)
The Voltouri will finally get what they deserve
The happy and peaceful ending will have been earned. Add on!!!
It could have made the book longer (which is a good thing)