Over the past 15 years I have looked at numerous business plans, of all sizes, many industries, and of various debth of project, while managing a $1B venture fund. The biggest problem I had with most of the plans I reviewed was the plan was not a living breathing document.
For a business plan to work, beyond funding, there needs to be a clear goal and a living strategy behind the plan. That plan should become the bible of the business going forward. But a bible of a new kind. It is a living breathing bible that grows and changes as the company grows and changes!
The only two companies I funded that failed, failed because they didn't follow a plan, and would not listen to that. They both folded within two years.
When you write a plan, it should be a plan that you can live with, and one that you live!! As your company grows, so should the plan, and if your products shift, shift your plan!!