User blog comment:FairyRaven/Fairy Tail Chapter 315: Rosemary, Review and Commentary/@comment-4985657-20130118101415/@comment-99.226.148.82-20130118101732

If I had to guess, I'd say that Milly is Minerva's last ditch armor. Say, when Erza starts to go down her usual route of winning fights, Minerva will pop out Milly as an effort to kill off her momentum. So long as Erza is still in the fight, Minerva still has reason to keep her around. After all, in the King's scenario, better to gain six points (Erza + Milly) than one (Milly, then getting slaughtered by Erza)