The new Unity interface in Ubuntu since version 11.04 has a global menu bar. This menu bar shows a behaviour that I find very surprising, it hides the menu entries as long as the cursor is not above the menu bar.
When the cursor is somewhere else it looks like this:
and when you hover over the menu bar it looks like this:
I can personally think only of disadvantages of this approach, it makes it harder for me to directly move the cursor to a specific menu, instead I have to first mouse over the menu and then move horizontally to the target menu.
As this feature remains the same in the latest Ubuntu version it seems to be a deliberate choice and I assume Canonical put a lot of thought into it. So I assume there are advantages to doing it that way that I just can't think of at the moment.
What are the usability advantages and disadvantages of hiding the menus in this way?