I just saw another business plan with a mission statement, and I thought, "How '90s!" Mission statements can be useful, but they really have become passe.
A good business plan should focus on answering the questions investors, creditors and strategic allies ask most often. And "What is your mission statement?" is not one of those. Better to spend your time writing about your value proposition, your management team, your relevant market, your business model (how you'll make money!) and your exit strategy.
A credo or mission statement can be a good way to express your goal for your business and how you'll conduct your business. It can go hand-in-hand with your vision for what your business will become. It can be like a rock that you return to in a storm. But it really doesn't have to be in your plan at the outset.