The major thing is that Advent Children is a movie, a non-interactive medium. So it's ok and in fact necessary for them to decide what they want and then go with it.
FF8 on the other hand, is a game. And the chief characteristic of games that differentiate them from other mediums is that you have (or should have) an impact on what happens. I'm not saying FF7 did it perfectly (Persona 4 on the other hand did that part very well) but the choices you make does have an influence on the outcome. To not have that but still give options that seem like they ought to bring different outcomes is to missuse the possibilities offered by the medium.
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Ours is not to reason why, ours is but to do and die.