Since this is HN and most people here have written code, I compare multi-monitor setups to a text/code editor with a single panel/pane. You just don't do it! Sublime, N++, Atom, etc. all support multiple panes (split screen). Once you get coding enough you never go back to using a single pane.

Same thing with multi-monitor. As long as you are productive (unlike the author), more screen real estate is better (obviously, there are limits)

Personally, I like dual 27"s. Allows you to run 4 full size (think 8.5"x11") word docs at 100% side by side. Very nice!