Logically, nothing. Practicality, sometimes it is the small thing. When looking for my latest vehicle I was dead set on a hatchback for that extra storage if needed. I have a desktop, laptop, and tablet. Each has their place. You accept this already. I am excited over Surface (at least Surface RT) despite owning a iPad already.
Why migrate away from the "iPad" standard?
1) The freaking stylus. I get Jobs hated styluses because he never got over the period you were forced to use them. I get everyone wants to copy his "genius". Still, there are times when I want to write on my digital tablet like it is actually a tablet. Using my finger on an iPad feels like I'm writing out notes with a highlighter. Using a third party stylus feels like a crayon. Microsoft is recognizing some of us want to use pens. Maybe I'm wrong, but this is something that I haven't seen in Android or iOS yet.
2) Why I say Surface RT and not just Surface. Same operating system as my computer at home. I can use the same software. Sure, it may not be wise to install Steam and/or Photoshop to this thing. At least I know I can readily chance between them with similar environments with programs I use across each.
Why not an ultrabook?
1) I don't own one. My laptop was my main computer before I admitted I needed something with more power for my photo/video editing jobs. I see laptops as portable home offices. Ultrabooks are too small for my taste as a laptop. To me, Ultrabooks are like the GMS Caballero. Some look cool, but in the end if I want a vehicle with a bed I'd be looking at a truck. Not a car.
2) There are times when having an ultrabook might be useful. The Surface RT is a tablet that can become an impromptu ultrabook much easier than any ultrabook I've seen so far can become an impromptu tablet. I like that little keyboard cover. If needed, it is there. If not? Hey, just fold it out of the way. It has just enough form factor to feel like real keys instead of pecking at glass.
So to get back to a car analogy. I see the ultrabook as trying to be a GMC Caballero. The iPad and most tablets as simply being cars or compact cars with trunks. I see the Surface RT as a hatchback car. Sometimes you need that versatility.