Doesn't mean that you should never do those things, just that I generally find them overrated as your ability to gauge market sizes for non-consumption products is very limited (which is what we try to do almost exclusively, attract people who haven't used a product in the category before).
I flew blind in my first business too, but I didn't have the gut instincts built up over years of consulting, like you did. So my first business failed, for a variety of reasons that included the fact that the product didn't fill any real need. If we'd done a bit of market research earlier on, we might have figured it out early enough to adjust.
Now, of course, if someone follows all the best bits of "Getting Real", they'll have a better chance of figuring out these kinds of problems ahead of time, even without a business plan. But Getting Real is not necessarily all that easy for everyone to follow. You need to already be a moderately experienced entrepreneur to make the most of it, imho.
For the neophyte entrepreneur, then, I'd still recommend spending a couple of weeks putting together a business plan, simply as an additional safety net.
I always see the recommendation to spend a huge effort on business planning, etc., up front before you start. This can often lead to giving up on the idea, though. Yeah, you might say then that the person doesn't have the passion to be successful if that is the case but alternatively, I've found that with software, simply doing it and getting something working is what really generates the motivation.
Maybe business planning is good but it doesn't necessarily need to be the first thing you do and it's not worth it if the result is that you do nothing instead.
I'd argue to just do something first.
How do you know you are doing just fine? How much of the market have you captured? How big is that market? What if you had created your business plan 5 years ago and today you did triple your sales? Would that extra 2-3 weeks of work be worth it?
I'm pretty sure they just look at their bank accounts.
In all seriousness, there's doing fine and being greedy. There will always be a step to climb, a market to reach or revenue to grow. The goal is to do enough to enjoy life and be comfortable, not to be the richest man in the world. At some point, you can do "well enough" and not worry about what "could have been".
My point is this: Though they may have taken Jeff Bezos on for his expertise, Bezos made his investment to earn a return. You don't invest in private or public stocks in a capitalist world to do just fine. You don't start a business for that reason either. That's not greed, that's entrepreneurship.