Why does Unity hide the menubar?

Why does Unity hide the menubars by default, instead of showing them in the free space across the panel?

One of the design goals of Unity is to reduce the clutter of the desktop, another is to use space more efficiently.

We hide the menu by default in Unity because the menu provides no useful information to which you can refer just by looking at it, but it puts a lot of detail on the screen which is visual clutter. So, we've taken the view that the menu is there if you need it (by moving the mouse to it or pressing Alt) but otherwise isn't in your view.

Many modern applications are doing without a menu altogether, so in our view, this is a step towards the future, and it will encourage application developers to think about their interfaces and make them more usable by design rather than depending on the crutch of a menu.