I agree with your disagreement!
My primary setup is a desktop with two monitors. From time to time, I've wondered whether the second monitor is overkill. However, my most recent long stretch of using a single screen (by having to use a laptop on the road) has convinced me of how helpful a two-monitor setup is. With a single screen, there is a lot of window-switching when I consult documentation, whereas a dual monitor setup makes sure the documentation is always there and I don't need to switch between any windows (or virtual desktops for that matter).
Having said that, I'll share a few additional details that have helped me (though they won't necessarily help everyone else). Neither of my monitors is huge (by current standards), with the main one at 22 inches. I think a huge monitor would make it hard for me to easily maximize windows into a usable size. Also, I was glad to find a rotating monitor for my secondary screen, which I use in portrait mode, something that's helpful for long documentation. I think a third monitor would be two distracting to me. If I have to switch between reference documents, I'll just switch windows within the secondary screen. Then again, having a third monitor might be worth a try. If I ever try it, I might try having both the second and third monitor off to the same side, rather than to the left and right of the main monitor, since it'd probably be easier for to remember "For documentation, always turn my head to the right", rather than remembering which reference source is on which side of the main screen.