Windows Server Datacenter Edition allows many things to include Automatic Virtual Machine Activation (AVMA). Using AVMA, you can activate equal to & lower & older editions than the host. I can understand the reason to go with older editions if you have legacy applications or something similar which would be limiting. But, I can't quite wrap my head around why you would want to activate Standard Edition if you have the ability to activate Datacenter Edition. Is there some advantage to doing this? Any drawbacks of just activating everything at the Datacenter Edition level? Some special use case I'm overlooking?
From a feature standpoint there's no difference between the two except for AVMA. You would typically license Datacenter edition for your physical virtualization host (whether running Hyper-V or ESXi) and then deploy Standard Edition on your VOSE's because it has feature parity with Datacenter Edition (again, except for AVMA). AVMA can only be used on a VOSE as a guest. It can't be used on a guest to activate other VOSE's, so why deploy Datacenter Edition on your VOSE's? I've worked with a lot of companies using both Hyper-V and ESXi and I can tell you that I've never seen one that deployed Datacenter Edition on their VOSE's.