Let’s face it, there are a lot of good books that get turned into bad movies. On one hand, you might wonder: does it really matter? After all, the original is still there. The mere existence of the movie doesn’t alter the fact that the book is good, any more than the remake of Psycho diminishes the worth of the original.
The first problem is simple visibility. Books rarely become pop-culture phenomena, and those that do are usually nonfiction (or at least billed that way). But movies generally have massive, nation-wide advertising campaigns, by the end of which everyone knows about them. Pick any bad movie based on a book, and chances are more people will know about the movie. That’s a lot of people who could have experienced the original — or at least a good movie — who won’t go near it. (This is less of an issue with well-known source material. A new version of Hamlet isn’t going to take the original’s place in anyone’s mind, though a good one may, over time, supplant older Hamlet films.)
The second problem is that once one studio adapts a work, it will take years before anyone does another adaptation. (Again, this is less of an issue for established material. Returning to Hamlet, there was plenty of room for both Mel Gibson’s and Kenneth Branagh’s versions.) Part of this is contracts, but it also comes down to a question of perception. A studio is not going to look at something that made a dismal flop and say “We can do it better,” they’ll say “Oh, that flopped, let’s not try it.” They’ll wait until there has been enough turnover in the audience that they figure most people will have forgotten the flop. And an author who has seen his work mangled may not trust the next studio that wants to buy the film rights.
So yes, bad movies do matter — not because they diminish the original but because they distract from it. And they matter because they set back the process of getting a good movie made.