No matter how you try to explain, every decision comes with a risk which might work out or not. Its true that if the project didn't work out as planned, then its a free labour, its also true that if it does, that might be more than winning a jackpot so anyone who want to try it out should just ask himself one simple question, which of the outcome will I be able to live with if I take the decision to join in or not. And the case is settled.
It's like investing - in this case not money but time and ability. And whenever somebody invests in something and it's a legit investment it involves the risk that it might not turn out as successful & profitable. But if - it could be worth much more than the usual labor-hour-payment.
Because of the most important question: Nobody will die ;-)
There are many different points of view, but I think you should pay the developer at least if you pay the developer, you avoid problems in the future.
My opinion!
I posted this for David Siegel. So, no payment from me. ;-)
I don't even think it needs much discussion about this. Those who are interested can contact him. Those who are not are not.
