I don't think it's fair to say that a movie tie-in is irrelevant. Sure, some people enjoy Splash Mountain regardless of whether they like "Song of the South" or not. Disney has also built many great rides without a movie tie-in, like Soarin' or Mission: Space (although I'll take your word on that one, I'm too chicken to ride it!)
However, having a movie tie-in does draw crowds. There's a reason that Disney's newest ride in Hong Kong is called "Iron Man" and not "Guy in a Robot Suit". The big hit at
Disneyland is "Cars Land", not "Southwest Driving Experience."
When you have a hit movie to base a ride upon, it automatically increases the potential draw for that attraction. Disney (and Universal) want to draw as many people as possible, so if they can draw in some fans of the movie who otherwise might not have noticed or cared, then it brings in more people and makes more money.
You can take that a step backwards and say the same thing about the movies themselves. Many movies are created as original screenplays, but many are based on books. The idea is that fans of the book will go see the movie, thus increasing its draw. Harry Potter became a successful "World" at Universal because it was a successful movie franchise first. Harry Potter was a successful movie franchise because it was a successful book franchise first.
So the real question, then, is this: what would have happened if Disney had just made an "Alien Adventure Nighttime Attraction" at AK, instead of Avatar Land? Would it have as much "buzz" around it? Probably not. But is the extra "buzz" worth what Disney paid for it? I'm not so sure.
Here's an interesting thought. Entirely speculative on my part, but still interesting.
"Avatar" is about a human man who gets magically transported to a magical world filled with giant, colorful, human-like aliens. He goes on some amazing adventures trying to save the giant aliens and stop the world from being destroyed in an environmental catastrophe.
"John Carter" is is about a human man who gets "magically" transported to a magical world filled with giant, colorful, human-like aliens. He goes on some amazing adventures trying to save the giant aliens and stop the world from being destroyed in an environmental catastrophe.
Perhaps Disney was already thinking about building a "John Carter Land" at AK. Six months before John Carter came out, the buzz was already terrible. So, they bought the rights to "Avatar" to hedge their bets. Surely enough, John Carter flopped, and Disney went full steam ahead with Avatar Land.