I see desktop workspaces as virtual auxiliary displays. I prefer a dual-display setup, and it's changed the way I use a computer. When I am on my laptop I feel extremely constrained by having only one display, so I make extensive use of OSX's virtual desktops. I never had the urge to use these extra desktops before changing my workstation to dual monitors, so for me at least that's the causality. Mountain Lion also makes it very easy to full-screen applications, and switching between them via a trackpad swipe is very convenient.