One shared menu bar at the top of the screen is supported by Fitts's Law, which says that targets at the edge of the screen are faster and easier to click:
Fitt's Law also says that closer targets are easier to click, and is concerned only with how long it takes to move the mouse pointer rather than how that fits into the larger picture of what you're doing.
Having the menu separate from the window breaks spatial locality and associates the menu to the computer rather than to the application.
http://en.wikipedia.org/wiki/Fittss_law
No comment on the rest.