I think it could be because some people love hierarchies, and others hate them; most are somewhere in between, and I would certainly expect those who are more hierarchically-oriented to prefer working with the virtual desktops than those who want a flat "everything at once" model. You can see this in how different projects' source code is organised too --- some are full of deeply-nested directories and very "branchy", while others have lots of files in one directory, or few files with lots of code in them. I'm personally on the "flat" side; I prefer there being fewer levels of hierarchy, with more on each level. Even tabbed browsing, which seems quite popular, is something I make little use of --- I'd rather open new windows so I can see them all in the taskbar among everything, and switching is thus a single action rather than two.