[arde]

This one wasn't one of the reasons I considered, but I find it interesting:

Suppose you upgrade any old OEM Windows XP or later that's bound to the hardware that you bought it with. Once you upgrade, you are no longer bound by the old license but by the new one. So you now have a full Windows 8 Pro, not OEM, that you can move somewhere else, virtualize, etc. That's what the license says, as far as I understand it (IANAL, but I have read it thoroughly).

Edit for clarification: even though you get a Windows 8 Pro, not OEM, it's still a Windows 8 Pro Upgrade. So yes, you can move it somewhere else or virtualize it, but it still needs to be installed over an existing and properly licensed Windows copy (XP or later).