Comment 3 for bug 732653

Hiding the menubar and only showing the menu on hover basically makes the most important benefit "A menu bar at the top of the screen. .... It has several benefits, most importantly being quick to use because of the large virtual target area..." a moot point, from <http://design.canonical.com/2010/05/menu-bar/> !

Even if user realizes that the menubar is hidden in the top panel, there would be no possibility of "quick use" (Fitts law ).
User can not know which menu item to hit, It will slow down the user in getting to the target menu. And hiding the menu for large screens just to save space is a very odd! ;-)