I had been using a Thinkpad T42p as my sole computing platform. It got annoying to keep tethering devices to it (external storage, sync my remote, sync my ipod, sync my phone, etc), and there were things I wanted to do that were always on (and the laptop, naturally, wasn't). so I decided to replace with a desktop to which I could attach all these things and just leave them there.
Then, my laptop could just be for mobility. And at that, the Air is perfect. I never used my optical drive while away from my desk on my old computer. I don't need hundreds of gigs of disk space, just enough for the projects I'm currently working on. I don't carry around a ton of USB devices and need lots of ports. I don't even need tons of speed: the dual core is faster than my old laptop. It's perfect, and it's light.