I get that, I think I've misled you with my comments. I realise that the build of a device can be its limiting factors, not mentioning issues like battery capacities needing to be matched to hardware power consumption, etc etc.
I meant generally speaking, like for example, getting the best GPU they offered in a particular range (say the Airs) meant ordering the most expensive model, which happens to come with larger capacity hard drive, and other extras you don't neccessarily want, nor wish for.
The MacbookPro 15", i5 and i7 models are a good example of this. In order to have the GT330M 512mb GPU you have to order the i7 model, costing more money. It would be nice to have a GPU upgrade option so one could choose an i5 with the upgraded GPU (the 512mb instead of the i5's 256mb) and pay less money for it. I'd personally want the best CPU/GPU available, but I'm not fussed about the memory capacity, or disk capacity, as both of these can be replaced for considerably less money in most cases by doing it yourself.
I've always found all models (except the mac pro) to be very closed with hardware upgrades - The specification I'd prefer always lies between the two extremes, and I feel like I've got to spend more in order to have the important hardware items I want, and accept others I don't neccessarily want in return. The same was true when I purchased my iMac a couple of years ago.