So yeah...why a MacBook Air? Why not a MacBook or something with more expandability options?
As a component they are more expensive to get at but more importantly it's about space. The nice thing about the macbook Air's motherboard is how narrow it is. The other MacBooks have small motherboards too but they spread out at the connector end. This still makes them narrow enough to fit under a standard keyboard but that loses favour as it means you can't get the keyboard (a desktop keyboard that is) low enough to be ergonomically comfortable. It can be done of course but then you have to expand out the back and you end up looking like an Amiga or something (hmmm...thoughts enter my mind of using an A600 case.....)
Laptop keyboards manage to do it of course but then laptops have a nice wrist rest area to rest your palms on so height becomes less of an issue. Plus laptop keyboards are hell skinny. The Air's keyboard, even with with light sensor keys and all that, is still skinnier than the standard slim Apple keyboard and for a desktop keyboard the Apple offering is positively anorexic. Anyone that remembers using the old laptops where the keyboard was at the front (and the laptop was 3-4cm thick) will remember what an ergonomic nightmare it was trying to type on the damn thing. I remember one manufacturer got around this by making the keyboard detachable until Apple figured out if you shove the keyboard to the rear of the top case you gain space to rest your hands.the rest is history.
But I digress.
A motherboard about half the width of a standard keyboard is nice... just like the motherboard of the Air. Sure a few extra expansion ports or a CD drive would be nice...but then it wouldn't be as cool as having an entire computer tucked under a standard keyboard profile :)