True, but I don't think anyone really buys an MSDN subscription just for installing a copy of Windows. I haven't looked into the actual licensing terms, but my MSDN subscription (not the cheapest version) only allows me to claim five license keys per Windows version through the UI. If you need six licenses just to make up the cost of the cheapest MSDN subscription, you're probably using your MSDN subscription incorrectly.

Generally if you have an MSDN subscription, you're using it for other products like Visual Studio, and the free copies of Windows are just for helping you get a development environment set up.