Writers can write the most brilliant, character-driven and tightly plotted script of all time and it can still turn out a mess--the director (and, in a number of cases, the studio) can and will re-write as desired, regardless of the initial quality of the script. Writers for film are rather powerless in that regard.
Conversely, a script can be all kinds of messy before a director re-shapes it into something far more palatable (if memory serves, something like this happened on Casablanca, though it could well be another film from that era).
In the end, blaming the writer for how a story turns out in a film is not usually fair (unless the writer is also the director). A screenwriter is far less responsible for the final outcome of a film than a novelist is for a novel (or a non-fiction writer is for whatever book she's writing).
Joss Whedon would likely agree.
He wrote the story for Alien Ressurection, and blames the production crew for making the story less than what it supposedly was. I think he felt the same way about the movie of Buffy the Vampire Slayer. (which, if I recall correctly, he also directed.....update....I was wrong, he only wrote it).
I don't know for sure. Whedon had some great stuff with BvTS the series, and with the all too short lived Firefly, but I don't know if A:R and movie BvTS are a result of corporate and production meddling, or if they were really his faults as a writer.
Then again, The Avengers did kick ass!
