The counter argument to that is "Great idea - what other value are you going to bring to the table?" For me, being asked to sign an NDA is often a signal that they don't have anything else unique to bring to the table, which is usually a recipe for failure. Instead, there should be some other reason why this idea will only succeed if you do it. Some good ones are personal domain expertise, access to partnerships, access to distribution channels, design/marketing skills, etc.
If all you bring to the table is an idea, than to be honest, the developer probably doesn't need you.
