George R.R. Martin has written:
Everywhere you look, there are more screenwriters and producers eager to take great stories and “make them their own.” It does not seem to matter whether the source material was written by Stan Lee, Charles Dickens, Ian Fleming, Roald Dahl, Ursula K. Le Guin, J.R.R. Tolkien, Mark Twain, Raymond Chandler, Jane Austen, or… well, anyone. No matter how major a writer it is, no matter how great the book, there always seems to be someone on hand who thinks he can do better, eager to take the story and “improve” on it. “The book is the book, the film is the film,” they will tell you, as if they were saying something profound. Then they make the story their own.
They never make it better, though. Nine hundred ninety-nine times out of a thousand, they make it worse.
Amen to that. The full blog post is here.