As usual, I'll give Patrick of Bingo Card Creator as my example. He has been completely transparent with us guys about what he's doing and how he's doing it. We have a complete blueprint for how to compete with him. None of us are going to bother, because it's a very small market opportunity and Patrick's business relies upon generating lots of data, lots of inbound links and the Googley momentum that comes with it.
Imagine if his position was slightly different. Imagine if Patrick had discovered that a very specific group of people were prepared to pay him good money for very trivial software. There are plenty of examples if you go looking for them - software that took days or hours to develop but that sells for serious money because it solves a real business problem in a low-tech industry. The value in the business is the domain knowledge needed to identify the market need.
If you're selling a web app to nerds, then stealth mode makes no sense. You need to engage with your market as quickly as possible, and chances are that someone is developing the same product anyway.
If you've spent good time and money on researching potential market opportunities, you'd be a total idiot to give away the results of that effort just because it's the done thing. I love getting to see the innards of so many businesses, but it makes me cringe at the stuff people will make public.
I don't think many of us realise that the rest of the business world considers our culture of openness to be absolutely fucking crazy. Just look at Apple - not even Jony Ive is allowed to speak freely. We see that as weird and Orwellian, but the rest of the business world calls it "not blabbing all our trade secrets to the press".