Actually it seems like a very good idea right?
In the same way u assemble desktops, you could get a cabinet(external casing), motherboard, and an LCD screen + other components which could be assembled together to make a laptop.
Only prob is whether there is any standardization existing. If each manufacturer makes components according to their own specs, then u cannot mix and match.
But who knows...maybe 5 years from now, we'll all be sitting and discussing branded laptops or assembled laptops..which is better?