But as I noted in the comments, the problem with Curtis' argument can be summed up in two words: data portability.
When files are just databases routed through apps, how do you ensure your data is backed up? What happens if a user-inaccessible database gets corrupted? Pulling everything from databases and manipulating it in-app is great from a design standpoint, but not from a portability standpoint. That approach divorces people from their content and makes them (even more) reliant upon designers to anticipate their needs.
Also, contrary to the multistep process for dealing with files Curtis outlines, with current Windows or Apple computers, all a user really needs to locate is the file; double-click it and it'll (usually) open in the correct app. Right-click and you can see the apps the operating system suggests for it. Drag it to Dropbox or a flash drive and you have a copy. None of those things are terribly difficult.