I am currently mentoring some MBA students for a Business Plan Competition. I gave them my perspective on preparing a business plan including differentiating between theory and practice since there seems to be a wide gap in real life. Many of the companies I have worked with only worry about creating a business plan when they need one to raise capital with a VC. In theory it should be a decision making tool and one that is dynamic and updated regularly. In practice, that rarely happens.
Wikipedia has a great article on Business Plans and Dilbert--of the comic book fame-who actually has an MBA from Berkley, is used in another very good article, with a shot of humor:
Five Criteria for a Successful Business Plan in Biotech. The Five Criteria are: Management, Management, Management, Management and LUCK.