I think you misunderstand my point. When a 2D filmmaker focuses on one image element it is easy for the audience to follow and thus the filmmaker can tell their story.
However with a 3D environment one needs to search for the focus point that the filmmaker is presenting to you. All of the other points are out of focus, but the point they want you to focus on will be out of focus as well unless you find it.
So when that trick is used it makes the entire 3D experience difficult for many people and simply impossible for others.
An alternative to focus which might work better for a 3D environment would be a contrast change so that the element the filmmaker wants to highlight is much brighter/darker than the rest of the scene. The viewer's eyes will naturally move towards that element and focus appropriately.