User blog comment:BlackWizardZeref/Fairy Tail Chapter 382: Alegria/@comment-24748439-20140501015948/@comment-7418318-20140501105257

Having all 12 keys doesn't automatically mean she'll be overpowered and sideline everyone else.

Apart from that, as far as arc end-fights go, she only actively and directly participated in the battle against Hades, so there is plenty of room to enlargen her role in team-fights between the current being side-lined and side-lining everyone else you fear.