Other mistake: Emmett states that the Cullens "haven't hunted in weeks", meaning all of the vampires should have black eyes. However, all of the actors still have their golden contacts in. This is especially noticeable on the Blu-Ray.
Other mistake: When Edward and Bella are playing chess on their Isle Esme, Edward takes one of Bella's pieces but puts his own piece down on the wrong square.
Answer: They want Alice because she can see into the future, which would be a significant advantage to the Volturi. Aro destroys other covens, sparing the one member who has some special ability.
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