I generally agree - but the only thing I'll say to defend stealth startups is that sometimes it makes sense to keep quiet until you have something to show people. Not for competitive reasons, or because you're worried someone's going to steal your idea, but because you want to stop your idea from being judged out of context or before it's ready.

Once it's ready (enough) to be shown, at least then people will judge it fully baked, instead of basing their opinion on the elevator pitch.