Even Storm becoming the head of the school has nothing to do with the main storylines (The Cure and Phoenix). That could have been cut and it would have made no change to the story. She's is a completely throwaway character: nothing she does arises from character motivations, she just does stuff because she's there and they have to give her something to do. Giving her more screentime in X3 actually contributed to the poor quality of the film, especially from a writing/structure perspective, because it was at the cost of devoting more time to a lead character (Jean) with an actual arc and storyline. It was a complete waste of film to have her do anything in X3: they should have just given her an effects sequence and a couple lines like in the other films.