You know, I think a hidden point in all this is that fundamentally, part of what movies do is try and control the audience's attention. It's the entire point be

You know, I think a hidden point in all this is that fundamentally, part of what movies do is try and control the audience's attention. It's the entire point behind carefully choosing camera angles and focus in the first place. The movie-maker is trying to inform the audience: "These are the people you should be listening to, this person's reaction is what matters, this object is significant to the plot..." If you can't control focus, then you are presented with a much larger scene. You (the viewer) would be choosing who and what to pay attention to, removing control from the film maker, and unless this additional degree of freedom is compensated for, the experience would go down. The way you tell the story as a result would change dramatically. You can't just tag on "3D version available!" to a movie and expect it to work, it requires a different way of telling a story.