It raises some interesting questions though. Is author creative control good or bad? I suspect probably the reason for being able to exclude the author from the filmmaking process is not just so the director and producer don't have to deal with another person (though I'm sure that's a major factor in many cases) but because novelists have expertise in a very different artistic presentation than film. Text and visual mediums work differently. What works on paper can often be tedious on film.
I've no specific examples of creator/author input ruining a film adaptation. So perhaps my theory, if correct, is more a fear filmmakers have than an actual reality.
I think, however, that not being able to make the crossover is becomeing less of a problem. Our culture is becoming more deeply grounded in in visual mediums. There is a lot of crossover, in both directions, in writers of comics and writers of novels, for example (Peter David, Neil Gaiman, Kevin J. Anderson, to name just three successful writers of both). One could argue that one reason so many superhero movies were complete crap before the last ten years was that the creators of those comics had no input to the films. Marvel's turned that around. DC not so much and DC films are starting to suck worse. Comics are already a visual medium though, so this may be less of a problem. There are differences one has to be aware of however. A comic can have absolutely no text and still be a great story. These days, a completely silent movie would be extremely difficult to pull off.
