The Hyper-V WMI interface is open to anyone to use. It is built-in to the core Windows OS.
For example: SCVMM actually uses this interface.
WMI is the defaco-standard API for Windows. It is the Windows implementation of CIM and thereofre actually based on standards and is not going away anytime soon.
The datacenter toolkit is just a bundle of resources, it is not an API. http://www.microsoft.com/hosting/dynamicdatacenter/About.aspx
It is for those folks that want to roll their own front end for Hyper-V.