Well, segmentation or not it's certainly worth making the Pro light and portable if they can achieve that without compromising its power too much. They seem to believe they can, or at least get close enough (I mean perhaps a larger/heavier Pro could have run to 32GB while maintaining acceptable battery life).

Whether it would be better to make a cheaper Air than the cheapest new model is a separate question. I'll just say that Apple has always gone the premium route (and the limited-range-of-models route) and there are advantages and disadvantages to that.