You may have already seen William Martell's much-discussed post on how the intriguing script "Nottingham" became the unintriguing movie Robin Hood. It's a classic showbiz story where an idea starts as one thing and winds up as something totally different; Ridley Scott plays the Jack Buchanan role from The Band Wagon, the director who listens to the writers' pitch, claims to love it, and then takes a lot of time and money to do something else.
What I like about the post is that while it can be seen as a "protect the writer's vision" argument, and a lot of screenwriters' advocacy groups (the ones that sometimes seem strangely wrong about what screenwriters actually do) are framing it that way, it's not really that kind of argument. Well, it is to a certain extent, since he talks a lot about movies he wrote that didn't wind up on screen the way he wrote it, but let's say that's not the only argument he makes. The interesting part is the argument that the producer of a movie should do his job and not let the production get out of control.
A producer's job is vaguely-defined, but it definitely involves putting the project together and making sure it doesn't end up being worse, or more expensive, than it should have been. The story of how Nottingham became Robin Hood appears to be that the producer was so deferential to the star and director that the movie got delayed for years, went way over budget, and wound up with a script that was completely different than the one the producer bought.
Traditionally, on a big Hollywood blockbuster movie, that's not supposed to happen. Once the producer has the script he wants and casting has been done and pre-production has begun, it becomes very expensive to make big changes. Which means that either the director figures out how to make the script work for him on the sets and locations that have been agreed upon, or he gets fired and replaced by another director. It's one thing if the director is attached to the film during the development of the script, and can make or suggest changes then. But in the case of Robin Hood, Scott came in with a release date already set by the studio, meaning that the project was at a point where delays are expensive. And at that point it becomes the producer's job to stand up for his judgment about whether a script is ready to shoot.