You compare the battery life, but not the screen to the macbook air? The screen is higher resolution, has a better coating, and is touch. Other than that, it's smaller and lighter (around 20%, actually), and comparably priced ($1299 for 256GB/8GB surface, $1299 for 256GB/8GB Air). It also has a micro SD card slot, to add further provisions.
This isn't a tablet competitor, it's an ultraportable competitor.
This isn't a tablet competitor, it's an ultraportable competitor.
e: made a bit more concrete