In statistics, there is this thing that we call cluster analysis. It's where we try to find different groups among the data that we are working with. Currently, when it comes to computer use, there is more than 1 basic group of use cases. There is heavyweight application use, but much (if not most) of computer time is spent solely in the browser, possible with some music playing. It's good that Google has decided to create an OS that is designed to specifically cater to this new use case.
No, Chrome OS isn't going to be your new desktop OS, it's going to be your netbook OS. Netbooks are designed to be portable internet portals, and a super-lightweight OS that's specifically designed for them is just what they need. And when a device debuts with it next year, I'll likely get one.
Furthermore, this shows Google's dedication to making the internet a better place. Between Go, SPDY, Chrome, Chrome OS, it's pushing toward HTML5, and it's various web-apps, it is becoming a better place.