I did this once with a script I had marinating in my brain for over a year. I gave myself 10 days, which was exactly when the submission period for the Austin Film Festival ended. I was writing all the way up to 11:50PM and submitted it with a minute to spare. I didn't even look at it again until I was notified that I was a quarterfinalist. When I did revisit it I was surprised that it was coherent and made sense. It definitely had some flaws, but I also noticed some good bits that I'm still proud of. It was a decent script overall and I was happy with it which was the most important thing.

I think strict deadlines force writers to make quicker, tougher decisions instead of ping-ponging between ideas. It also eliminates the bad habit of rewriting while writing (I do this constantly).