I dont agree with that. Then again I think SE and D does this kind of stuff on purpose cuz they get their kicks in looking at our theories.
I just kinda like to think about what I think would make the story stronger. Malificent and Pete sacrificing themselves for Sora is kick butt. The thought of her just taking control of the heartless is slightly repetitive. Besides, I think in the case of Oogie Boogie, we see that her power is somewhat...weakened.
Who knows. Would be cool to find out though.