Why would you run more than one operating system at the same time? An actual question.
Because I want to run different applications that only are available, or work best, in different OSes. For instance, Citrix GoToMeeting is only available for Windows, iPhoto is only on MacOS, etc.
I've always got half a dozen windows open to different tasks -- it's how I work. Email in one, a few browser windows on different sites, a Word doc, a spreadsheet or powerpoint, GoToMeeting, Skype, a text editor, a few xterms to the Unix boxes, etc. Back in the bad old days I had to either be dual-booting all the time (lost productivity) or have an array of machines on my desk and a KVM switch. Using VMs (virtual machines) is the perfect solution.