From a storytelling point of view, yes. You have to do things differently in a game than in a movie; adaptation is an art, and a tricky one at that.
The problem was that Spirits Within really wasn't Final-Fantasy-anything. For God's sake, it took place on Earth. Earth! In the future! Seriously? These are the people that gave us worlds like FF6's and FF7's, and the best they could come up with was post-apocalyptic Earth invaded by alien ghosts?
Where was the medieval/steampunk fusion world? Where were the crystals, where were the black and white mages, the airships? Where were the brave, sword-wielding warriors?
Where were the chocobos?
I wasn't really looking for any kind of adaptation of an existing game, but I was hoping for something a little closer than a pretentious scifi B-movie. Square slapped "Final Fantasy" on the product to sell it.
And yes, I realize Advent Children didn't have black mages or white mages or crystals, and honestly, no, I don't remember any Chocobos in it, but you know what? At least it didn't take place on Earth and at least Bahamut wasn't rampaging through New York City. I liked Advent Children because like every Final Fantasy game, it removed itself from our world.