This is so true. Never underestimate what the script actually does. It gets the film from "Fade In" to "Final Fade Out".
Yes, films have been made without a formal script but these are often highly experimental in nature and require a great deal of expertise from the Director, Actors and especially the Editor. A more structured script has details everyone involved with the production can see and understand and thus will lead to less hassle come production.
Great topic, James Woodland! From what I've heard, I wouldn't go into production without a finished script. Some of the movie won't make sense, reshoots, etc.