Multiple monitors are often significantly cheaper than one large one. You can also aim the two at you more accurately, so you don't have to change what distance you focus at as much. The productivity study was short-term; how about long-term? Certainly having things too wide means you spend more time mousing to those edges, but how about eye strain due to viewing angle? > That optimal number, for the vast majority of…
IMO, the real question is using widescreens appropriate for a dual monitor setup. My work setup is two 4:3 aspect ratio monitors which I think works much better than two widescreen monitors because it reduces the time it takes to move the mouse. For text editing I suspect three 4:3 ratio monitors is more useful, despite providing a similar number of pixels as two widescreen monitors, based on how maximizing windows operates and the pain of trying to split a window across two screens.
However, when working with images, having a large unbroken screen is extreamly useful. So a 2560 x 1600+ monitor paired with a lower resolution scratch space seems to work better than two evenly sized monitors.