Should films of books be made if they do not remain true to the original story?
I don't think it matters. The film is, as Nearly Sane points out, a different thing to the book. Films should be judged on their own merits. There have been films of books that I wish hadn't been made. The 1984 Dune springs to mind but my problem with that film is not that it was not faithful to the book but that it was crap.
There have been films that have improved on the book by being "not faithful to the original". My favourite example is probably Jurassic Park. The plot of the book is much more flabby than the film. Also, Bladerunner has a much better narrative than Do Androids Dream of Electric Sheep, in which pretty much nothing happens.
Jack Reacher is a film that is interesting in this respect. I saw it before I had read any of the books and I thought it was a perfectly fine crime thriller. All the fans whined about how Tom Cruise was nothing like the Jack Reacher they imagined in their heads but they are really just complaining that Tom Cruise isn't six foot five. Well, so what? If the defining feature of a character is his height, he's a pretty poorly drawn character.
Disclaimer: since I saw the film I have read all of the full length Jack Reacher novels.