Makes sense. But, then, why do professional teams keep great secretiveness? Is there another kind of (commercial) reason?
Although I remember I saw a development blog for Super Smash Brothers Brawl...
Everyone I've met and talked to that has a great idea they won't talk about [I used to do this when I was a noob] has been inexperienced and unable to pull it off largely because they don't understand the investment involved in realizing such an idea/vision.
Big companies do it because they are scared [wrongfully so] that the public will associate their company/brand with unfinished works due to the shoddiness of their alpha and then their PR bang will be greatly diminished when it comes time.
At least that's how I see it...
I'm not sure if it's always 'wrongfully so'. I remember one time at E3, Jade Raymond was giving a live demonstration of Assassin's Creed, and there was some glitch that made one of the guards fall through the wall in slow motion. This triggered a storm of posts on gaming forums about how glitchy and terrible the game was, followed by a bunch of games journalists pointing out that the game wasn't finished, etc.
I don't know how much of an effect that ultimately had on AC's sales, but it was certainly memorable. But I don't think that is a danger for indies unless you're releasing a demo.