Do you remember the tower-breaking game on Kongregate and iOS that Angry Birds cloned? Or Backyard Monsters that was the original Clash of Clans?
Except perhaps the dev with the initial idea? Stealing someone else's idea might be widespread, but it doesn't make it right.
Zynga shows me exactly why I don't share my ideas early on: A company with more money and more resources can bring the idea to market much faster than a 1 or 2 person shop.
I think the people that keep spreading this idea have the resources and money to 1) create their own ideas quickly and 2) take someone else's idea.
At some point you have to put your ideas/implementation/whatever out into the open (you might even have to advertise its existence). At that point, anyone better resourced than you can probably just mimic your work.
What alternative does anyone really have?