[martinced]

Lots of people do not know what they want. Hardly anyone knew they "wanted" FB because nobody actually thought they'd be needing it. Less alone pay for it.

If it's really "killer" (not as in: "I'm creating a social app where people hunting cows with bows can use their smarthpone to share videos of arrows piercing bows in real-time") then people may not know they'll actually want this.

I don't disagree that having other buying apps using your idea is good in that it shows there's a market.

But then you don't "Have a great idea". Because you're idea simply is "My idea is to copy another product".

Not that I have anything against it: it's what I do for a living : )