Think of the VM as an empty machine. You need windows or linux etc running inside them todo anything. So if you want to run windows then you'll need it.
I've run four operating systems at once on my MBP (OSX, VM'd windows, VM'd Redhat, VM'd ubuntu) for work before.. so it's not just for running windows.
Thanks - seems too expensive as I just want to use Napster on my Mini without using the web version.