I think that most don't want to see the movie because of how vulnerable and helpless it makes them feel. It similar to a person not wanting to see a video of their mother getting raped or beaten up and you being helpless to interfere - it just gives you a sick feeling.
I certainly think that we should never forget and that it's important to keep showing videos and audio recordings just so it is fresh in our minds.
I don't know if many people can take 2 straight hours of it though.
That said, the film will probably contain some quotes from the Hijackers like "let's twist the noble faith of Islam to our own evil ends" and one of the passengers fighting back will be a Muslim, etc.