How has the role of the screenwriter in films changed over the years?

Some screenwriters are nothing more than "hired guns," where some guy writes a screenplay and another writer, like Robert Towne, comes in to "clean up" what was originally written, so that more sense can be made from the screenplay. Someone in Towne's position doesn't add to the script as much as he changes it so that it's easier for a director to shoot the project. Very few times will you see a film that has only one screenwriter on it, unless the director is attempting to take all of the credit. Usually, one person writes it and another person "polishes" the script so that it can be made into the film. That is why you see someone with a story credit and then someone else who gets a screenwriter credit. Very few times does the guy who has the story credit actually get the screenplay credit, unless it's more than one person working on the project.