Approximation is not what it's about. It's about integration. Write that google doc and store it as a local .docx or .odc. Edit that 20 MB raw picture in a browser app and store it back to the disk, instantly, then feed it to a traditional desktop editor. Write that Python script in a Web IDE, store locally, and upload it to an Arduino.

All those use cases and more can be accomplished if the Write API is limited to a sandboxed local storage with a configurable size limit per domain. I would hate to see all the gains made by sandboxing the browser fly away by allowing it to access any file on the user's system.

I think the idea here is that you'd whitelist files/folders on your actual filesystem for use with the app. This sounds fantastic for me. There's so many useful web based tools that are a pain to work with because you have to manually sync files to and from them.