Why would you bother doing that? In a word, security. When you elect to restart your system into Safari, you're effectively placing the Web browser into a sandbox. When it boots, your system will give any users with physical access to your machine the ability to surf the Web. But that's it. Users won't be able to access the system's files or applications.
And thanks to Lion's new auto-save and application restoration capabilities, users that slap their systems in Safari-only mode will be able to restore back to their full desktop exactly as they left it. Since Safari mode runs off of a system's recovery partition, you'll still be able to access the Web and research new methods for fixing your system should your primary partition suffer some catastrophic upset.
The comparison to Chrome OS stems from the fact that Google's operating system runs entirely Web-based: The browser is the primary method for interacting with the system. There's no underlying desktop layer to speak of.
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