Eh, but she wasn't NECESSARY is all I'm saying. They could have easily taken her out and just written off the process of Guardians turning into Sin as something that Yu Yevon himself controlled, and the story would literally have lost next to nothing because of it. They just added her in because they needed a big climactic boss at that point in the game. Unlike Jecht, who, if removed, basically destroys all character motivation for Tidus, or Sin, which, if removed, would more or less change every aspect of the game. Sure Auron has a grudge against her, but he could have been killed some other way, as he never really makes a big deal about her killing him and all.

Actually, it would have been more poignant if the Unsent or some other representation of Yu Yevon himself took Yunalesca's place in-game, as it would help to build up to the final confrontation against him, and make his role in the game more apparent, thus making his last-minute appearance less forgettable.