Q:

Why is Desktop Unity using the global application menu?

It was announced in another question that the desktop version of Unity will keep the global menu by default. Here are the facts:

The global menu was introduced into UNE to save vertical screen space because at Netbook resolutions the vertical space is limited.
On a modern desktop with a high resolution, there is ample vertical space making this unnecessary 
On the announcement of UNE global menus, Mark Shuttleworth himself said the following:

"There are outstanding questions about the usability of a panel-hosted menu on much larger screens, where the window and the menu could be very far apart."
The benefits of a global menu don't seem to carry across to a high-resolution desktop and instead seem to bring draw backs (increased mouse travel, large distance between the menu and its associated window).
The other worrying factor is that applications seem to be moving away from having a menu bar, and instead of innovating on this and defining new guidelines for moving away from the menu, we are giving it prime place right at the top of the desktop. If applications continue moving away from the desktop we will have an inconsistent experience concerning where to locate application related options/tools depending on which app you are using (e.g. Chrome).
Finally, the current global menu bar implementation doesn't work for all apps, and doesn't even work for all apps in the default install. This means that the default desktop implementation will be inconsistent.
So, there are a bunch of reasons why moving to a global menu is a bad idea, so we need some pretty convincing arguments for why it is a good idea.
What are the reasons for the global menu implementation in the desktop version of Unity?

A:

On the one hand, we are making menu's "bigger" and "easier to hit" by using the edge of the screen, as noted in the first answer. On the other, we will actually deprioritise them, by using the panel primarily to show the application name (or window title) and only showing the menu when you mouse towards the panel, or use accelerator keys related to the menu. In that way, we're leading the trend of making menus less central to UI.
Ted Gould blogged some research he did on the use of menu's. Informally, his findings support the idea that menu placement is less an issue as we use them less than we tend to think we do.
This was an important question for us and one we believe is settled in a way that's supported by research. We supported the original pitch to make the global menu a feature of GNOME, which was unfortunately rejected.

A:

First, one of the same reasons Mac OS has always used a global menu applies equally here. One of the key principles -- tautologies, really -- in UI design is that "bigger" things are easier to "hit" with a mouse.
What is less obvious until one thinks about it, is that things on the edges of the screen have infinite height or width (depending on whether they're on a horizontal or vertical edge).
By keeping a menu at the top of the screen, the user only has to "aim" horizontally -- vertically, they need only "throw" the cursor to the top of the screen.
Second, the idea that "Desktop" Unity doesn't need to conserve vertical space is just silly. In recent years, I've used wide-screen 17" laptops with 1920x1200 displays, often with an additional external monitor of the same resolution, and I find myself wishing for more vertical space all the time.
Screen real estate is always at a premium -- not just on netbooks.
Finally, with regard to the remark about usability issues with the distance between the menu and application windows on large screens, I'm not at all certain where that's coming from. Mac OS has been getting along just fine for years, including as my primary desktop environment. There are undeniably implementation issues with the global menu's actual functionality with some apps, and inconsistent or poor design practices have certainly led to some applications relying overly much on menu bars, or laying their menu bars out in strange ways, but these are not fundamental problems with the global menu paradigm.

