That's exactly how I do it, Greg! I almost never plan ahead with stories, unless I think of a super-cool event that could happen. Or if I'm at school, where these things are required. Stupid brainstorms! If I wrote a story using a brainstorm, kept the copy, traveled back in time and wrote the story without using a brainstorm, there'd be a major difference. The one written with a brainstorm would be extremely bad. Reminds me of my writing exam. They gave you 5 minutes to plan, 20 to write and 5 to edit. In the planning time, I just stared at the blank area of 'planning space', then during the writing time, I just wrote away.
Actually, I find that writing spontaniously makes everything I write a lot better. Even research essays. Trust me, everything I write that's been thoroughly planned turns out to be a pile of junk.