If you have a really bad script idea for a movie, you should edit it with FCP rather than iMovie - it automatically generates a better movie for you using some complex algorthmic scripting and wavelet compression. Apple spent a lot of time developing these technologies for making movies better, hence the increased cost of the software. If they'd used FCP on Battlestar Galactica, for example, they would probably have guaranteed at least a $75m box office.
However, if your movie has a really good concept, you should probably go ahead and just edit it in IMovie and save yourself almost a thousand dollars in the process. You can spend that money elsewhere, such as in lighting or craft service. You won't see the difference up on the big screen, and your crew will have gotten much fatter into the bargain.