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For me its not just about multitasking. Its about distraction. With one window per app on a big screen (eg Windows 8), its just too distracting to switch. I have to go to application switcher, choose appropriate window, and everything looks different now. However, if I've typical two windows side by side of even overlapping, the switch very less distracting...and of course quicker.

That said, I do use a tiling window manager on my 13" laptop for work. I mostly use full-screen mode, but it works well for me, because switching is very fast.