OT: I can't think of a single film that is arguably "better" than the book/playscript on which it is based. People don't make movies out of bad books (I promised myself I wouldn't mention Twilight at this point), so it's not so much that a good film will "improve" the story, but that it will hopefully offer a new and appropriate interpretation of the original material. Case in point Apocalypse Now: Conrad's Heart of Darkness is an amazing book, but would seem outdated, boring and inappropriate in today's culture, particularly for its inherent racism. Francis Ford Coppola managed to transfer the basic anti-imperial discourse of the novel to a contemporary issue, the Vietnam War, and still keep much of Conrad's powerful imagery intact in his fantastic cinematography.
It's worth also remembering that film and literature are very different mediums; as film appeals directly to our senses with sight and sound, while literature relies on engaging our imagination. This is by no means a criticism of anyone in particular, but audiences accustomed to film and television may not have had the chance to develop the imagination necessary to get the most out of a more subtle breed of novel, leading them to argue that the film is "better" because it succeeds in communicating where a novel would not. There is also in Hollywood a lack of appreciation for the nuances of certain types of literature, particularly tragedy, so what you end up with often are "happy" endings that satisfy more people but utterly destroy the author's vision. My Fair Lady and Breakfast at Tiffany's are two classic examples. What happens then is people read the book and declare it to be "worse", because it doesn't provide the expected happy ending.
It's worth also remembering that film and literature are very different mediums; as film appeals directly to our senses with sight and sound, while literature relies on engaging our imagination. This is by no means a criticism of anyone in particular, but audiences accustomed to film and television may not have had the chance to develop the imagination necessary to get the most out of a more subtle breed of novel, leading them to argue that the film is "better" because it succeeds in communicating where a novel would not. There is also in Hollywood a lack of appreciation for the nuances of certain types of literature, particularly tragedy, so what you end up with often are "happy" endings that satisfy more people but utterly destroy the author's vision. My Fair Lady and Breakfast at Tiffany's are two classic examples. What happens then is people read the book and declare it to be "worse", because it doesn't provide the expected happy ending.