13" Retina Macbook Pro vs. 13" Macbook Air

These machines are really similar, and the idea is that the MBP would be more powerful. But given that an Air could be equipped with a i7 processor, and assuming the MBP can do the same what other reasons would there be to choose one over the other?

The MBA is lighter obviously but the MBP wouldn't be bad either. The old MBP was 2.1kg, I'd have to imagine the MBP-R would be lighter than that.