The safari-only mode in Lion can be used as a security precaution against Mac. After the Find my Mac is turned on, Apple allows only the guest user to use Safari and no other user can log on. This technique can help users find out where the MAC is currently located (IP address).
The Lion browser-based model is very similar to Google's browser-based lightweight operating system Chrome OS.
Apple offers this browser mode to users in a different way than Google. Google is working with manufacturers to provide low-cost laptops that use only the Chrome OS. But Apple is unlikely to adopt the same strategy as Google for Mac OS X Lion, which offers a nice sandbox model to make Lion a safe and anonymous Internet tool.
Apple doesn't seem to be competing with Google's Chrome OS (the cost of Apple hardware is at least twice times that of chrome), unlike the safari-only model. But for the above reasons, adding a browser mode to Mac OS X Lion still make sense.