What are the practical reasons that would cause you choose one over a Macbook? I'm afraid I don't get it, unless that half pound less is really important.
It weighs less. 680g less than a MacBook. (1.5 lbs) It also takes up less space.
Yes, weight and is important when it comes to laptops. I travel with my MBP and now I want a MBA, because the MBP is heavy to carry around for hours at the time along with other stuff. The MBA would be less of a burden.
If you don't mind having a heavy computer, good for you, save some money.