This is just an API for encouraging the use
of proprietary plugins ("CDMs").
(Using a formulation by Ian Hixie)
This seems to be related to Bug 20963 (EME is technically incomplete). Specifying the CDM would likely allow the API to be removed or reworked so as not to encourage the use of proprietary plugins.
Discussed on the telcon:
CDMs by design are out of scope for this document. If someone wants to design a different solution to protected content that doesn't push CDMs out of scope they are welcome to. The EME spec is not designed to prevent "plug-in CDMs" but will likely constrain there scope more than generic plug-ins.