User blog comment:Miskos3/Fairy Tail Chapter 419: The Message of Fire, Review/@comment-77.194.131.61-20150208112159/@comment-7418318-20150208201528

@Sane: Sure, but there's a lot of room between being able to use it and mastering it. If it's initially rather weak, she would still use mainly her Celestial Spirit Magic, but with a little backup in case her keys get lost or to back her Spirits up.

If Lucy only managed to get really powerful with it right before the final fight, it wouldn't hurt Juvia's role, since I highly doubt that Juvia will play a big part in that final fight.