I wondering if you have used new 13inch mbp and 15inch mbp or not ?
I used them in Apple store, performance difference was negligible.
I've played with them both in the apple store, and you're right - the difference is negligible. The problem will come when people start to use graphic intense software such as Adobe CS, Rhino 3D, 3D Studio Max, final cut etc.
I agree the13" MBP is powerful enough for most people's needs but as a PRO machine it should be able to do the tasks that professionals need. The macbook is apple's consumer line. If that was given that the aluminium unibody, most people would buy that as most people are only worried about how it looks, not how it performs.