And the surface is better?
For example, you cannot do programming on the go, without being connected to a server that handles your development tasks.
So the surface has to be better than the MacBook Air for work, or better than an iPad as a tablet.
Otherwise it's just a compromise that is worse at both.
[edit: the downvotes are pretty indicative of the nature of this thread given that all I've done is ask valid questions to which I don't know the answer]
With a Surface, I can go full developer mode in the desktop, while making use of, if you prefer, the old style applications.
I can detach the keyboard, switch the Metro (Modern) mode and make use of the tablet as a digital notebook thanks to pen input and handwriting recognition.
Finally, I can go put the pen aside and just use touch input.
And the ability to have a docking station made specifically for the Surface is the cherry on the top of the cake.
With a Mac Book Air, I am stuck on the classical desktop mode.
The question is - does it do any of those things better than the competition.
When in "classical desktop mode" it needs to be better than the MacBook Air, otherwise for most work it will be a worse experience regardless of whether it could at any time transform into a tablet.
On my (employer) corporate Win7 laptop, the virus checker sometimes lights up the hard drive continuously, making the IDE almost unusable on it. My tablet feels pretty damn snappy.