I think it's a widely accepted fact that, as a guy with an idea and a laptop you are worth zero without proper execution. So you have everything to gain by sharing your idea.

Things get trickier as you build value. There's still plenty of information you'll benefit from sharing, but it starts mixing with stuff you definitely don't want to share.

I see a common trait in a lot of successful companies: they found a healthy balance between too open and too closed.