Jason,

Good points. I've always viewed a Business Case as a mini Business Plan. As such, the process of documenting assumptions is a powerful exercise the vets the quality of the idea.

I'm experiencing this now with a startup client. As we develop the bus plan and financial model, some of the founders' original assumptions are not holding water and forcing us to reorganize parts of the solution. Which leads to the next point --> a Business Case establishes constraints for the Solution Architecture.