The character dynamics that can be played off. No other character can have the same dynamic with Jean or Cable that Maddie has bc of their unique history. You keep looking at her as a function as opposed to a character. If they can have Kwannon and Psylocke coexist when Kwannon was barely even a character 30 years ago and their history is even more convoluted, then I dont see why Maddie would be so problematic to have around