There is some question about whether business plans are a good use of your time and energy. They do eventually go on a shelf and there they gather the dust. I don't know all the answers, but I can share my story. I started my firm on October 15, 2007. For the months of July and August, I spent every spare second working on a business plan. I started with a template from http://www.score.org/ and went from there.
I have always been a creative writer, but the template I used would not let me skip around the parts where I was most weak. Marketing, marketing, marketing. I can sell a client on what I do, but can I identify who that client is and make them come to me? Truth be told, I have been fortunate in referrals, but without the plan, I would have been less focused and really gotten myself in trouble early on with some work that came my way that was not in the "plan". As it was, I turned some of those jobs down, and actually earned some respect from my peers who thought I was really "brave". I look back now and laugh - it was a risk, but I had a plan, and it is in my head and I stick to it. As an aside, I did end up hiring a marketing consultant who was really helpful and who kept me on track. Hire to your weaknesses!!!!
I think the point of the business plan is the process of figuring out what you can do and what you are going to need to buy. I also believe that the discipline required to write the plan is an important indicator of your commitment to the business. Running a business is not easy - I often consider it my second marriage. In the business plan stage, I was planning the wedding - with so many things to think about and do, and yes, a lot of trepidation about what this type of commitment would mean going forward. But on the day I left my "job" and woke up, I knew what to do first because I had the plan. And just like in my marriage, we just went forward and did it. And we keep going.
Oh, and the important information - I did secure funding with my plan, and the loan officer relayed his supervisor's words, "They have to have a comprehensive plan or they have NO chance." I am not sure that is true, but it sure helped me.