I have to disagree. Fang's involvement in Vanille's story is critical to her feelings of guilt and shame. These feelings come to a head in Pulse and Vanille's eidolon awakening (side note: she gets hers so late).

She also becomes Ms. Exposition several times in Palumpolum and Pulse, and bonded with Lightning during that time. Heck, she even has an optional scene with Sazh discussing his role as a father (a character she hasn't really spent much time with too much).

She becomes even more important in Lightning Returns. Basically, while she isn't so much involved with the personal team dramas which basically made chapters 2-7, she propels the game along into the second act.