Honestly, I feel better about it now. Beating Perospero was an obvious reason to keep her around, but now we see she's around for a different reason... I just don't believe that Oda brought her along for nothing. I also don't think her beating Perospero would have been satisfying, since Perospero did nothing wrong (before the raid).
She was the emotional tie to the minks, a face for that side of the alliance
She was also a device used to showcase sulong