I think no matter what kind of person you are, having a business plan on paper can only help you. I am also an "on the fly" type person. But what I do is, when I have an "on the fly" moment, I just incorporate it into my written business plan.
If you have a business, you have a business plan. It just may be in your head, and not on paper. You come up with an idea (in your head), and you put that idea into action. That's why I think putting down the idea and plan of action that you have in your head onto paper is a good idea. It doesn't have to be structured the way a "traditional" business plan is, if you don't want it to be. But I think putting down your idea (plan) on paper can only help you in the long run. I like to call it "backing up your hard drive". You have all of these great plans in your head, so what would it hurt to put those down on paper. You can always modify it if you need to. It's your plan.
Also, as you get deeper into your business(and alot more busy), you may have ideas that take you away from what you really wanted your company to be or to do. You can go back, take a look at what you orginally intended, and see what you want for your company from an objective point-of-view, rather than an idea that may seem good because you just thought of it, but isn't really what's best for your business overall.
Don't misunderstand me. I'm not saying everyone should have one, or that the only way to be successful is to have one. Just that I don't really see any negatives to having one. If you are successful, why stop there? What if you could be more successful? What if a business plan could actually help you to be?
Websailor,
A good tool that I have used is "business plan maker" and "business plan maker pro". You can get either in almost any store that sells computer software.
It asks you different questions about your company and puts your responses in a business plan format. It can be as loose or as structured as you want it to be.
It sells for about $40. Since you say you don't really have business sense and need some structure, I think this would be perfect for you.
