Comment 27 for bug 930148

It just seems natural to me that if the desktop is empty, then you may well be wanting to launch an app or do some work of some kind so it makes perfect sense for the launcher to be there, ready and waiting.
Then when you have launched your app, it moves out of the way so you can fully concentrate and maximize the screen space.

The autohide behaviour at the moment seems pretty iffy and unpredictable. I find the launcher takes more effort to show than it should when I WANT it to show, yet when I overshoot the back button in chromium and nudge the edge of the screen, the launcher shows instantly.