If most users are driven by practicality (in other words they upgrade only when they need to) why are there lines outside the AppleStore every time a new product is released?
Explain to me why several of those in line for the 4s had iPhone 4's already? (I never said "need", I said they upgrade for practical reasons. Almost nobody "needs" these devices.) The average Joe cares more about specs on a piece of paper than they do design. That's why we have things like the megapixel and MHz races. It's marketers responding to the market.