Nobody seems to be considering that if you "just get an iMac and Air for the same price", it's still two computers. If I just plug into a display, it's exactly the same machine just presented larger and there's no need to rely on a cloud to make sure I have the right things on the right unit. In terms of performance, I believe certainly with the retina model at least that they are pretty much on par with each other, and the iMac is therefore sort of the budget option for people who are willing to always use it in one place. If money is no object, laptops and after-purchase displays are the definitive way forward because it's like having two computers with the ease of use of having just one.