If all the models are made from the same materials and have the same features then what would set them apart other than size? Not enough to distinguish them. White or Black iPhone... White or Black Macbook.
But... they don't have the same features.
MBA lacks internal optical drive, Firewire, upgradeable RAM
MB lacks dedicated GPU, illuminated keyboard, Firewire 800, ExpressCard, matte option etc
There's still enough to separate the lines. Just