... if you know how to use virtual desktops and shortcuts. You can't look at two screens at a time, anyways.
One use case I can understand is having a 2nd monitor for checking stock prices or checking for a certain event. Other than that, I don't see how it contributes to "productivity" while working or coding for example.
P.S: Tiling WM users may understand this post more
Imagine you have to cross examine 3 papers on your computer, you have to take notes and you have to input information on an excel document for your research paper thats on another word document. Try doing this on 1 screen.