I always thought so, at least it gave the project a structure to work from and a game plan for other colaborators to follow and contribute to. Apparently, unless you're looking for independent financing, you don't really need one. Still, I'm kind of fond of having a document that we can shape together as we build a world around the script/story, it gives us a place to plan, build and structure.
When You Don't Need a Film Business Plan - Baseline Intelligence.