That depends on the individual, for you ultraportable is how thin a laptop is (though would a 15" MBA just as thin as the AIR be considered ultraportable?), for me is about how compact it is. Each to their own, personal preference.
In regards to a bag, I can put a 11" laptop in a bad that is only 11.5"

, but if you always have a big bag, thinner can be better as it takes up less width, and fits better between books etc.
I think ultraportable refers more to the smallest screen you are prepared to have in a laptop. For some 11" is just too small, for some 13" is the perfect size, again personal perference. Also weight is a huge factor. its nice to put a laptop in a bag and not feel like u jsut put in a brick in it....