One could argue that if a browser is your only environment, that you'll be spending the most time in, then why wouldn't you want the best option for running said browser? The way I see it, if you're going to be working in a completely remote/cloud capable environment, then a powerful, fast, beautiful screened machine to work from makes the most sense.
My current job gave me a Dell Latitude E series machine for work. I work exclusively in AWS. The screen on this laptop is so horrifically bad that I practically get sick looking at it. Compared to my personal (2010 era) macbook pro, or the Retina MBP's I've used at my last two jobs, there is no comparison. I'd much rather have a lightweight powerful little machine that I can ssh from and have some sort of IDE or text editor to work on my AWS projects then worry about a big bulky crappy laptop that is overkill for my work.
Obviously I realize this is MY current use case, however I'd be willing to be that the majority of dev/devops type folks on HN have a similar type of use case. AWS, Heroku, just about any PaaS, Saas, or Iaas, etc... would take advantage of a machine like this beautifully.
Oh and the last time I had an Air (right before upgrading to the rMBP), My use case? Chrome, Chrome, Terminal, RDP, Chrome. That was at a PC Game dev/publisher company where I administrated all windows based desktops, mac laptops as well as all servers of misc variety...
Sooo.....Is this machine useful? I think it is, and I am excited to check it out and may look to getting one.
One more thing, I too am one of the lucky Cr-48 owners and LOVED the concept. Biggest reason I hacked it to load ubuntu? SSH. Had the plugin existed back then? ChromeOS ALL THE WAY BABY!!!
/rant.