The popular ultrabooks and all of the ones I've seen in real life are 13". My comparison is between 10.6 and 13, which is a pretty big difference. Regardless, I can see how this is arguable.
Definitely, if you want a 13" screen, the Surface is a poor substitute. But by the same token, if you want a 15" screen, a 13" Ultrabook is a poor substitute. It's a tradeoff between portability and screen size. If you're willing to go down to the low-end of screen size for sake of portability, the Surface Pro looks very competitive with 11" ultrabooks. If you're not willing to go to that screen size, then the Surface Pro isn't really an option.