The primary reason is that it's underpowered. A number of my co-workers used them until they got really frustrated with resource starvation slowing them down, and they switched to MBPs. Starting up Firebug on an Air on a sufficiently complex page or a webpage in Chrome that uses Flash can bring the thing to its knees.
The second thing is that I'm a big fan of developers developing in an environment analogous to their deployment environment. Chances are pretty good that as a Node developer, you'll be deploying on Linux, not on OS X. OS X does a good Linux impression, but there are a lot of errata that you're going to run into. Running a VM on an Air is going to really tax it. As the "Linux user" in my group, I tend to know what kinds of deployment issues the software will have, while my co-workers struggle with a host of OS X-specific bugs that I never contend with.
I use a pretty oldschool setup - headless Linux server that I work on via SSH/Samba. It's not for everyone, but it lets me work on my desktop (custom built beast of a Windows machine), Chromebook, Linux laptop, MBP, or even from more exotic locations (I've fixed production bugs from my phone before, including running test suites). Having your full development environment a) mimic your production environment, and b) be accessible from anywhere that you can get SSH is amazing.
That said, the MBA is a great machine if you're going to be moving around a lot. The form factor and weight are significant benefits, and the battery life is remarkable given its size.