Having two monitors is a huge deal if you're copying something from one program into another. One thing that comes to mind (perhaps not particularly common of a developer) is writing up a project report from spreadsheet data, email reports, paraphrasing a contractor's report, copying a lost file that has only been printed and scanned (not common, but it does happen), etc. In this case, having multiple desktops on one monitor would be not much more than an annoyance in these cases; it's a little like saying "why do I need desk space for one fully open book when I can just look at one at a time, half-open each book to look at, or just put it on the desk behind me?"