Well I just like to have documentation and other stuff on the side monitor (SSH terminal into the server, mail client [umn or reddit]) and code on the main monitor, that way I don't have to switch around and at least working on GUIs with only one monitor must be horrible (well writing GUIs is always horrible, but it'd be worse

)
For photo editing I don't see how more than one monitor would be especially helpful - I mean you could let bridge open on the second monitor, but editing is a mostly sequential task and the border kills the idea of using both monitors for one picture.
For gaming it depends - some people love it, others don't.. for racing games it sounds perfect, I'm not so sure about FPS..
Desk space.. well I've got 2 24" monitors on my desk and a third monitor would make it a bit crowded I think.. 2x1m