Over at Six Colors, Jason Snell offers some historical context on the heavily-rumoured 12" MacBook Air:
If Gurman’s reports are accurate, this new model pulls the MacBook Air line away from the MacBook Pro. In fact, it returns the MacBook Air to its roots—as a product full of choices that we consider crazy at first, because they’re out of step with conventional computer design, but that will appeal to a target audience that doesn’t actually care about those de rigueur features.
I owned an original MacBook Air too (the curvy first model, in its second—slightly speedier—revision), and loved it in spite of the inherent design compromises. This was before iTunes Match, before DropBox, before iCloud, and using it as a primary computing device meant making some pretty tricky decisions about what you were going to load up on that pokey SSD. Nevertheless I considered it a no-brainer: I had lugged my laptops around half the globe for the best part of a decade (first a PowerBook G3, then a white iBook, a 12" PowerBook, and finally a brace of Intel MacBooks) and though each delivered a different balance of convenience and functionality, I found myself inexorably moving towards valuing portability over power.
This was of course driven by the changing nature of my own employment (less design and coding, more writing and researching), but also by the dynamics of hardware and software: Moore's Law and Mac OS X seemed almost conspiratorially intent on delivering more functionality for less with each refresh. By the late 2000's it seemed faintly ridiculous to be hauling around the equivalent of a desktop system when all I really needed was a note-taking and communication system. Less than two years after adopting the Air I was well on my way to replacing it with the first iPad.
This same dynamic leads me to agree with Jason Snell: The MacBook Air needs to return to its original position as an outlier, a statement of intent, a herald of things to come. In a couple of revisions the MacBook Pro will be as thin and light as the original Air was back in 2008, and pretty much every notebook manufacturer has a svelte model in its range (though inexplicably I still see people hauling the kinds of kit that make that original Lombard G3 look small). Apple can let the new Air point the way towards the kind of Mac that'll be mainstream in two or three years, and at the same time make the neither-one-thing-nor-the-other hybrid tablet/notebooks look ridiculously clunky and compromised.
I haven't bought a notebook for myself since the first IPad shipped, but if the 2015 12" Air is all that it's rumoured to be then I could easily see that changing..