It's not like a house where everything is planned from the start, but more like an evolving Formula-1 car that is tweaked and tuned with the user inside of it.
I like that methaphore!
However as you metioned yourself: building (great) software is hard. Why put the reponsibility at the (Powerpivot) users desk? It's hard enough for users to analyze conformed data in a (traditional) Datawarehouse let alone build a powerpivot solution.
Dirk