I disagree heavily - Having a secondary monitor with documentation / work plans on it is enormously useful as you can glance at it without having to do anything more special than turn your head. In fact, I would go so far as to say that three monitors is optimum since you can have your code on one, the output of your code (continuous tests/hot reload/etc) on another and documentation on the last allowing you to see a…
My desktop monitor is a big one though (and 4k) and that helps. On it I do many of the things you say you do with multiple monitors.