I spent yesterday morning at the Fantasia Frontiėres Co-production Market listening to movie pitches, and pitching my own feature, Souvenir. There was a distinguished Polish director who told us that he couldn't pitch his movie because, as Antonioni told him, if you pitch your movie, you won't make it.
I'm in the opposite camp. I think pitching is one of the most valuable creative tools in your toolkit. You can learn more about your own movie in a few hours of preparing a pitch and delivering it than you might learn in weeks staring at the outline. Prepping a pitch really forces you to think about what you want to promise the audience to get them to invest their time and money in your story. What goods are you promising to deliver? Then when you pitch it, the people you're pitching to immediately let you know if they're buying the goods.
Then, you go home and rewrite, based on your pitch. As I write a story, I tend to focus on the linear aspects of the story: the plot, the revelation of the characters, the twists and turns towards the ending. I have to go back and make sure I'm taking my time with the emotion and the spectacle.
In our case, they dug it. They laughed and clapped in all the right places. We're promising fast cars, a seductive djinni, and people dying in inventive, horrific ways, so we're right at home in Fantasia.
Whether your script is finished or not, there's a lot to be said for pitching the hell out of it. If it's not finished, you'll know better what you want to write. If it is "finished" (if any script is ever finished until you shoot it), then you'll know better what to make sure you've written.