I disagree, the 'no file' approach is the forward looking one, beyond DropBox type thing. Why sync files to local machines when they are already accessible through the cloud? that's the 'leap'.
Edit: I think I understood the rationale but it's blinkered. It's Google's business to find out why such transitions are difficult or unattractive to users and to smooth the way. To answer your question, why sync, an example is data portability. While most computer users have never heard the term, they do have an intuitive understanding of the kinds of problems that can happen when you don't have a local copy of your stuff, as almost happened with the Google Video screw-up. Whether problems like this are real or permanent isn't the most relevant fact, it's that people experience them.