The business plan can be reviewed but should not be written by a person outside the management team. The business plan should be an expression of the entrepreneur's vision, not that of an outside writer. Sophisticated investors know the difference between a plan that has been written by a professional outside consultant, and one that is written by and pronounces the vision of the founders of the business. If your management team does not have the expertise to draft the plan, expand your management team to include those people who fill in the management gaps. The business plan is a management tool, which you can use not only to direct the business and raise capital, but also to attract management talent.
Many investors go right to the resumes within a plan before reading the plan.
They want to see a well-rounded and experienced team of full time people, and complementary consultants and advisors. The better your management team and support personnel, the greater the chance for success. You can take a fabulous idea or product and mix it with an inexperienced management team, and create a formula which will have difficulty succeeding. On the other hand, with an experienced management team and an average product or idea, the chance of success is much higher. Solving problems takes experience, and people like to see that experience mixed in with the entrepreneurial founders of a company.