Hmm...then how about Mana series characters? Or SaGa series characters? Keeping Dissidia only based on Final Fantasy series (mostly the originals, but that would also include FF Tactics given their link with the Ivalice Alliance and...FF Crystal Chronicles >.<) would reinforce the idea of a in-world crossover. Adding KH would sound moderately reasonable, except that you'd be locking out the rest of the RPGs developed by Square-Enix. There would be no reason why Dragon Quest characters cannot be there (at least even Erdrick/Loto), as a nod to their original competition, or add the main characters from Estpolis/Lufia given that they're now published by Square-Enix (though the game will be brought by Natsume in these parts, so not sure how it'll work out in the US).
That would be the core of the reason behind the "hate": either you make it limited to one series, or you make justice to all S-E series, since there's no particular reason why Kingdom Hearts should be a pick over other games with influences from Final Fantasy.
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