I'm pretty sure Grace has three tails specifically because I wanted her to grab Damien with two of them and pummel him with the third. I enjoy thinking of ways inhuman forms would move around and fight, and I like the potential of three tails.
Granted, I'm not certain how they'd work from an anatomical standpoint, but let's all agree not to worry about that. Besides, we let Tails gets away with spinning two tails to fly in the Sonic games, and that's just silly.
Mild spoiler warning: Nioi's earlier assumptions were, apparently, NOT correct. I never followed through on any schemes to make Grace's shapeshifting less potent.
I was about to type that the entire point of it would be concern for Dragon Ball Z type power fairness, but that doesn't work, because DBZ was never concerned with keeping powers equal. If a good guy was too powerful to keep things interesting, they just found a way to keep them out of the fight for a while. In any case, given that EGS generally isn't about who can beat up who, it's okay for Grace to be a powerhouse.