98% of venture capital firms say that business plans play an Important or Somewhat Important role when they look at new ventures (according to a study recently published in the Canadian Journal of Small Business and Entrepreneurship).
What's even more important than the business plan document, though, is the business planning process. I have seen many companies come to new and important realizations about the key success factors and obstacles for their concepts by working through the business planning process, by conducting good research, and by projecting financials. They make smarter decisions faster and more cost-effectively through business planning than they would by launching and trying things out.
So while a business plan is incredibly important, it's the process, not the outcome, that matters most.