Screenwriting is different from every other form of writing, such as novel writing and poetry. While I can imagine someone writing poems purely for their own expression or self-enjoyment, NO ONE writes screenplays purely for the experience. Screenplays are written to be filmed.
Or to put even a finer point on it - screenplays are written to be sold.
A novel or a poem is a work of art unto itself. A screenplay is merely the beginning - the blueprint for the film. If you want to work in Hollywood, never forget this.
I don't fault anyone for trying to make an artistic expression in their writing, but at the end of the day, this is a business and the sooner one embraces that, the easier their time will be. Writers like to get all huffy about their artistic integrity - and BELIEVE ME, as a writer I understand that impulse - if you're writing a spec script, someone is going to have to buy it.
So if your agent tells you, "You know, maybe you should rethink the graphic incest rape scene because it's going to scare off female viewers, which will scare off buyers," they're not necessarily trying to stifle your creative voice and tear the heart out of your story. They're just trying to make the script more appealing.
Don't assume take this post as advocating watering down and homogenizing your story. Strive to be bold, strive to be original - that can get you noticed too. But there will undoubtedly be times when rewriting in order to make a sale will be a necessary evil.
A good writer can take a story that means a great deal to them, and execute it in a way that will make that story meaningful to an audience. The difference between a writer with a sale and a writer with a spec often is that the former can balance their creative desires with the tastes and expectations of an audience.
Sometimes the most important question a writer can ask themselves is "Who will be interested in seeing this story?"