[Osmium]

Agreed that it's a net loss in legibility, but it does improve spatial reasoning. In a single window environment like iOS, there's a definite hierarchy: the home screen lies below apps which in turn lie below the control panel/notification window. Being able to see the blurred contents of the view below e.g. the control panel really helps reinforce that and stops the user getting lost in the OS, though admittedly at the cost of legibility. It's a fine line to be sure...