I like to use the term "Business Guide" instead of "Business Plan".
When I first started hosting, I didn't have a "plan" as such, but I had an idea and direction on things. Over the years I referred back to the guide on how I wanted the shape and direction of the business to proceed, but there wasn't an actual plan. I didn't have goals or markers that I needed to hit, instead, I had goals that would be nice to reach.
Of course, I ran my business very differently (much to how I run the business with my wife currently). Profits and Client Target Numbers are not on the forefront of any of our discussions. We don't answer to others, so our goals and targets more revolve around how much time away from a computer or vacation we want to take.
Certainly, if you're looking for investors, talking with a bank, or any other angle like that, you need a plan. How will you make money, how much time to make X, and how long to repay. But if you're funding the business yourself, and have limited shareholders, or a shareholder who just wants a piece of the pie and not dictating that they want a 9% return etc - then a plan really isn't needed, and a guide or outline is a better route.
It's not for everyone, but it works for how I operate. Of course, if I was to present my ideas on how to run a hosting company to an Economics Professor, I wouldn't pass the class
And crazily enough, it was for that reason that it was successful