So they can unify their entire laptop family range, which would be a blessing for Apple as a manufacturing company.
I mean, come on, the chassis for the current 13" MBP retina and the current 13" MBA are not that different at all in terms of size or weight.
So... why not offering just one single -retina- family: 15", 13" and 12" (or even 11") to accommodate to every user's need (performance, balance and mobile respectively), and be done with it? it would simplify their production, logistics and marketing.
Apple already follows a similar approach with their iPads (7", 9" and now rumored an upcoming 12"), all sharing pretty much every component except the screen. And they're following the same path with their iPhone 6: same phone, same components, but different screen sizes: 4.7" and 5.5".
To me, it seems like a good idea from a corporate point of view.