And yet it works.
I look at it in the opposite direction, in theory. If a manufacturer will not share some of the potential success of a project with you, from the incremental value that you add, then the design negotiation was not optimal. In practice it does not apply to all jobs. Some design work is just work.
Well put Scott. Some projects you get pitched have a dubious future from the beginning, other projects you realize from the onset that you are going to add a serious multiple to the sales quantity with your unique efforts. Try to balance the risk and the reward. Establish a history of adding value and be able to sell that to the client.
A mix of up front payments (skin in the game) and success percentage reward over time is the ideal.