The curve of evolution of your final solution works very fast in the first month and you have the feeling that everything goes fast.
But suddenly the evolution stops and you find in the middle of thousand of adjustments that makes you think: “butterfly effect box” is the right name for SharePoint.
So build a quick pilot and give the customer the perspective and the feeling about what they are going to see at the end of the project is very useful in so much terms for the solution building.