I can see where you're coming from and if you put it this way it makes sense to develop a BI solution by that procedure. However I still think that most business users don't have enough knowledge of Excel, PowerPivot and Database Concepts in general to be able to build a prototype. And then again even if they could, if all users build a prototype using different assumptions and definitions wouldn't it be really hard to for the BI Project Team to build a solution with all the functionality in those prototypes?