When I first picked up an iPhone, I had no idea how to drive it. The only thing that was obvious was how to get back to where I started. There was a button for it. The home button is a 'get me back to the start' route for the uninitiated. If you have to use a non-obvious gesture (and let's face it, most gestures aren't obvious until you know them), new users will not be able to use the phone without some basic tuition.