It doesn't (really) matter that he has no influence in the main story because he is not the main char. Why did you think they gave Balthier the line "I'm the leading man, who else?" It was the point that they were trying to make: the story from now one focuses on Balthier, Ashe and Basch. Vaan was merely used because he served as an excellent introduction for a new player, which wouldn't have worked with Balthier or Ashe or Basch.