2GB for the Air is not only economical, it's quite ample. Any owner of previous MBA revisions will say they never really cared for more RAM but rather a better hard drive.
Being that Mac OS X is based on Linux, it's much better able handling memory management. Not only that, but with there being no spinning, slower hard drive, any bit over 2GB that might be used as swap space will not be NEARLY as slow.
I'm not sure how fast RAM is compared to a flash memory hard disk as this, but I'm willing to be it's 70% as fast. That means when you do tide over 2GB of RAM, the computer will slow down a tad but nothing terribly noticeable. Most people won't exceed that limit anyhow and those that do already know to upgrade to 4GB.
Apple is trying to make the Air affordable to all walks of earth. The point is being minimalistic (physically and hardware-wise). For most applications you are not going to maxing out the CPU/RAM, so why include that much by default? If you think you're going to need it, upgrade.
What always amuses me is that if Apple had made it 4GB standard and added $100 the price tag there would be a bunch of people saying "I wish I could get it for a tad cheaper." Obviously, they can't please everyone.