Well, when I switched from a 13" MB/MBP to a 13" MBA, even though the footprint was the same, there was a phenomenal improvement in mobility - the MB/MBP was on average twice as thick and weighed about 50% more and the AIR allowed smaller bags, smaller accessories etc so there was a real multiplier there that its difficult to appreciate until you try it. A fantastic "only" machine for mobile use. It seemed effortless to always have with you.
My eyes are not the greatest and I was nervous when I first looked at the 11". But when I compared the 13 and 11 it was obvious the 11 offered another paradigm shift in mobility. All the things I felt going from a MB/MBP to a 13" AIR were even more so going from a 13" Air to an 11.
The 11" air is TINY and seems to weight nothing - like a pad of paper. Its form factor makes it extremely rigid and strong so I don't end up with keyboard marks on my screen. I don't have to take it out of my bag when traveling by air (its exempt like an ipad). It has all the power and capacity of the 13" in a significantly smaller package. If you can live with the small but excellent screen and the shorter battery life, I would highly recommend it.
And you are right about the increased tendency to always have it with you (because its so small and light and unobtrusive). If you carry it in any kind of bag with other stuff, you'll find yourself checking to see its actually there because its so small and light you often have to look to tell.