On the Mac, you see it all the time. Because it is actually designed so that it can be seen. The only reason you want to hide the directory structure as a low-level OS implementation detail is because you can't do anything else with it! It's so hopelessly convoluted that it becomes a low-level implementation detail, instead of a way for the user to interact with his computer.

To be exact, though, OS X lives a bit of a double life - it contains both the legacy Unix structure, which is hidden, and the new Mac OS structure, which is shown.