Just found this thread. There are some good points and also a lot of flamebait.
The issue is that a certain segment of the market needs 4GB of RAM. Those people - myself included - would love to have a MBA and would be willing to pay a premium for a 4GB configuration. The use case is clear: anyone who wants to run Windows alongside OS X via VMWare or Parallels virtualization. Boot camp is not a viable option since it requires you to shutdown OS X.
The annoying part about this thread is the number of flamebait posters who say "why would you ever want to run Windows" or "if you want to run Windows, why not get a PC"? This is just annoying and naive: many professionals need to run some specialized business software that only exists for Windows, and we don't want to own or carry multiple laptops. Been there, done that.
The more interesting question is the comparison between an MBA and an aluminum MacBook. They're pretty close in size, and the weight difference is smaller since the new aluminum model was introduced. The MacBook can be upgraded to 4GB and has a faster processor, and you can even upgrade it (yourself) to a faster hard drive. And of course, the MacBook is much cheaper. But it is still not as thin or light as a MacBook Air, especially when you consider the MagSafe Power Adaptor.
Personally speaking, whether I upgrade my existing MacBook to an aluminum model or to a MBA depends on the kind of computer I will have in my office. If I get an iMac for work, then a MBA will be powerful enough for on-the-road. But if I continue to use my laptop as my main computer, then I think I need something more powerful than an MBA. Until there is a 4GB MBA, of course.
Firewire would be nice, but it isn't a dealbreaker for me. Target Disk mode is nice, but you can restore a computer via a bootable backup.