Hmm, lets think about this for a moment. First off, Apple sets off to design a 15.4" widescreen notebook (or is it 15.2", I think I remember the Powerbooks having a 15.2" screen). So they figure out what the depth and the width are going to be, but wait, Apple has a goal set out for its new notebook....they want it to be 1" thin. This is what a lot of laptop companies set out to do, but they get to designing the laptop and realize that to make the thing 1" thin it will cost them probably 200 - 300% more money to make each lappy.
So they do the best they can and get the "next fastest thing" out on the market. Apple on the other hand has a goal and they spend time and money making sure that they meet their goal. They are not really worried about cost because they know that they have a loyal following and they want to make the best product they can so that they can maintain their little share of the market. So is it really marketing, yes; but only because that is what Apple customers have come to expect out of Apple and the company does not want to disappoint (and they want to be that much better than the other guys of course).
Why do I say this? Well, if you look at what Apple did with the iPod Nano then you would understand. They discontinued the best selling and most successful mp3 player ever made because it was as good as it could be and they replaced it with the Nano, which improved on many of the mini's shortcomings.
Apple does a lot of things for marketing, but they are mostly driven by the fact that their products are different, well made, reliable, and cutting-edge for the most part.
Thank you very much I'll be here all week