My time is not free. When I think about how much time it would cost me to shut down OS X, the boot up Windows, then shut down Windows, boot up OS X, just to use an app, the cost of more memory and a license for virtual machine software is minimal. I run Windows apps on my work desktop all day (an iMac). I wouldn't get anything done if I had to reboot to swap back and forth all day.
Now, some people might use Windows infrequently, or exclusively for long stretches enough to justify not using a virtual machine. But for others a vm is the only way to be productive.
Amen.