My wiring makes me the absolute worst-case example of getting by by the seat of my extemporaneous pants. If I can walk in and wing it, why prepare? Preparation is dull, boring, tedious. Dull.
For me, this just-in-time approach peaking in college. I walked in to an open-book test on Numerical Methods without studying, forgot my programmable calculator, and both figured out the material and ran all the calculations by hand on the back of the test. Damn! Nothing feels as good as success when you pull off something like that. Clearly risk and adrenaline play a part.
Unfortunately, winging it doesn't cut it. The lesson I quickly learned, once I entered the real world and the stakes were higher, was that preparation wins out because preparation amplifies talent. Even mediocre talent. And in a competitive world, where the only two grades that matter are win or lose, you can lose just as easily to someone who's less talented but prepared as you can to a talented hot-shot, especially if the talented hot-shot thought he was going to figure it out on the day of the test (presentation, proposal, whatever...).
Yesterday, at 6pm, I presented my business plan to a group of angel investors. I could have shot from the hip. I know the material and I wrote the plan. But there were going to be other businesses presenting and I wanted our plan to stand out in everyone's mind. It was a 12 minute presentation. Over two days I went through the pitch, beginning to end, at least 40 times. You do the math. I could have done that pitch on a unicycle (even taking into account that I would have had to learn how to ride the unicycle while doing the pitch--by the seat of my pants, of course ;-).
Preparation pays off in other areas, as well. Would you go to a doctor if you knew she was going to figure it all out while working on you? Okay, the kidneys--those are the bean shaped things, right? I wonder where they are? Imagine the incandescent chaos that would result if all of our fire-fighters learned everything they knew on the job. There's a certain machismo in technical circles (I'm a prime example, so I know of what I speak) around the ability to figure out what you need to know by the seat of your pants. There's also a certain sadness that sets in as you watch a good project blow-up because the guy in charge of designing the backup solution didn't know what he was doing prior to learning it as he designed yours.
Why do so many start-ups fail? After failing a few times myself and watching a boatload of others fail, I can tell you with absolute conviction that in the majority of the cases it was because the people starting the business 1) didn't know how to do XYZ and 2) didn't bother to learn XYZ in advance of trying and failing to get it right. Replace XYZ with plan, sell, develop, deploy, hire, etc. and iterate that formula enough times and you go under.
There are times when it's necessary to wing-it. But save it for the times when you are actually caught unprepared.