So what, then, is the definition of an ultraportable? To many users, it is quite a bit more portable than a MacBook. I'm not sure what you carry your laptop in, but there are many, many people who need to carry around folders, spiral-bound notebooks, papers, books, etc., along with their laptop, and thus will have a bag at least as big as that, which is big enough for a MacBook Air. For us, a smaller footprint will make no difference whatsoever, while thinness makes room for more books, notebooks, etc.
A smaller footprint may be your definition of ultraportable, but it's not everyone's. Thinness adds a lot of portability for many people.
So far, most ultraportables have been characterized by a small footprint, which means a small keyboard and small screen. Apple realized that there is a gap in the ultraportable market for those who want portability without sacrificing keyboard and screen size--those who want to not notice that they're carrying a laptop while its in their bag, and then not feel like it's undersized at all when they open it up. And that's the way Apple went with it, which won't appeal the fans of a small footprint, but is perfect for many others.