Okay, first off "Plot Protection" is a horrible argument. If a story is so weak "Plot Protection" is your only argument as to how the heroes can possibly win, I have no Idea how you can enjoy the games story.

Also, you say you watch her absorbing the Stars. If she could absorb all instantly as you seem to claim, why can you see any stars getting absorbed at all? They should all be gone in a moment.

Finally, given the fact that the heroes were able to fight her at all, means that she can't absorb all time in an instant. Meaning there is a period of battle there her ability does her no good at all. Not even a little bit. Even Neo-Exdeath suffered from this same problem. Neither can make use of that ability if someone appears before them to fight.

Others, like Kuja and Kefka, use their full power at will.