That is very well stated! I think movies can " relay a version of the author's world to a different demographic," perhaps even an aspect of the story even the author was not consciously aware of -- an aspect that was merely latent as opposed to consciously developed.
On the other hand, very often adaptations of novels to film is done poorly, such as the lame Sci-Fi adaptation of the brilliant Dean Koontz novel (starring Stephen Baldwin), for example. If one was made aware, before hand, that the film was based on a book, this movie would have "ruined" the book for her. In other words, the temptation would have been to think that the book was substandard as well.
I happen to believe that NOT all novels CAN be successfully translated into film artistically. Some novels should remain as they are; the structure of the story itself, in my opinion, sometimes will not permit the successful transmutation of the book into film.