By hooking up Hyper-V to the Windows management OS, they get to leverage Windows device drivers. And this, in turn, ensures broader hardware compatibility.
Of course, this goal became unnecessary for Azure, where Microsoft controls the hardware. But Hyper-V had already been designed that way.
Certainly, Server Core is pretty stripped-down. The Russinovich blog says they reboot about once a month, but the Azure support blog says they reboot every 2-3 months.
http://blogs.msdn.com/b/wats/archive/2013/09/24/windows-azur...
"When this occurs, you loose connectivity..."
"While this type of proactive notification of planned maintenance is not currently provided, we encourage you to provide comments on this topic so we can take the feedback to the product teams."
Wow. They didn't notify at one point?
That's a pretty serious disadvantage over VMWare.
But I guess the limited attack area of Windows Server Core means many people can avoid applying patches much of the time.