The difference with Unity on Ubuntu is that they still made it easy to use Gnome2. For that matter, they made it easy to use Awesome, and I can almost guarantee a tiling window manager will never become the default window manager. The other benefit to this system, besides user choice, is that while the current system is in widespread use, people can still improve the old/different system, and the systems can steal ideas from each other, and everyone ends up with a better product.