When would you advise seeking out a business partner then? What role on a team would you consider to be a valuable contribution? Coming from the non-programmer side, I hope that you might have a better appreciation for this than one of us code monkeys.
I've always found, as a marketing guy, that I'm most helpful once the developers' initial prototype is up and running. That's when folks like me can start helping carve out the minimum viable product, identifying and communicating to the target market, and crafting the story to help get the product out there.
I've found that engaging earlier--and attempting these efforts sooner--is a mistake. It tends to cause more conflict than not. To me, engaging too early is like standing there and telling a painter "maybe you should use more green" or telling a musician "try a different chord."
I find I'm most helpful when I can engage with a developer who has realized the initial iteration of his/her vision.
Admittedly, this may result in a little wasted time and some backtracking. But more often than not, that's a small hurdle for the benefits of a) having someone realize their vision and b) reaching a point of being comfortable with, seeking, and accepting criticism and guidance. It just starts the whole relationship out on a much better footing.
