Me and my friend (both business students) tried to start a business with having the Business Plan as our first step. It went to hell.
Now we started with validating the idea - selling! It worked soooo much better. More people should try that instead!
Sure, a 200 page document that takes you six months to write that you work on to the exclusion of everything else? Yeah that's bad.
However a short few pages on the idea, the target market, some guesstimates on sales targets, expenditure and so on is a small piece of work that in my experience can be very useful.
It gives you a document to go back to as a bad line for progress. What is measured can be improved and all that jazz. Also, just taking the time to scope out your business, even at a high level, will make you think about areas of work that a founder might not otherwise consider until it's too late.