One PC screen is more than enough...

... 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