Compared to tilting my head to the second monitor, it is measurably faster to type ^3 (Control + 3) to switch to the space/virtual-desktop where my email window is open (~1.5sec vs ~0.7sec).

If one has to Alt+Tab or Command+Tab through programs, yes, that would suck the life out of productivity.

The only real benefit larger screen spaces (larger than a ~30 inch monitor) provide me with, are when I need about 4-6 open windows (maximized vertically) to switch between when dealing with large merges/patches. I haven't experienced any practical benefits beyond two ~30inch monitors.