The difference would be larger than between 13" and 15" MBP.
I assume that Apple expects that most people will want to see movies on their laptop, so they do not dare to omit the DVD drives and the overly large screens from their offerings. In the case of the Air, they offset the lack of an optical drive with lots of stylishness.
But see, the threadstarter just answers your doubts: There is a very vocal segment among prospective Mac buyers that do not want a clunky MBP or a stylish-but-compromising Air, but a true ultraportable. It is just that Apple does not consider this segment to be important enough.
How so? A 13" MBP that's just a little bit thinner and doesn't have an optical drive?
I didn't notice many people being concerned with thickness of MBP, it's the weight they usually complain about. If it kept the weight of Air, it still wouldn't have the optical drive, longer battery life or other features of 13" MBP. It would, however, use 1.8" drive where 2.5" would fit, and generally have a lot of free space inside (it could be designed to better dissipate heat, though).
Many people would still say it's a 13", and they wanted 11", while some say 10" is enough, still other arguing 12" is the sweet spot.
I'm pretty sure if Apple did this, people who like Air for what it is would say they broke it, people who consider it underpowered would say it's still underpowered and pathetic excuse for a computer, people who didn't like the footprint would still complain about said footprint, people who liked the design would say there's no point, people who say it's too expensive would still call MBP a better value. And people who want more ports and don't have footprint, power or price complaints, don't care about the design and are happy with current battery life would buy fewer laptops than Apple actually sells now.
The vocal minority you're talking about, based on my observations, wants a laptop lighter than Air, with a footprint of a netbook, more ports than 17" MBP has, power of 15" MBP and hopefully price of 13" MBP. Apart from weight, Apple is generally moving in an opposite direction. So that would be a "no".