Our Macs have two or three 24 inch monitors, which makes you realize just how dumb the whole thing is. If you are working on an application on the left monitor and need to access a menu item you have to mouse all the way over to the middle monitor and then back. Do this 200 times a day and you very quickly realize just how stupid the whole thing is.
Furthermore, the menu bar might not necessarily contain menu items for the application you are looking at because you happen to have clicked on the desktop or another application.
Yes, of course, you can mentally manage the concept. That does not mean that it is a good idea.
Linux, Irix, Solaris, Windows and other OS GUIs have gotten this right from the very start. The application is a self contained window and every instance travels with its own controls. No need to mouse across 72 inches of monitors to get to a menu.