The idea behind it all is "content control". That's why it is not to be expected that Microsft will ever release the specification for MMS.

BTW the home page for the SDP project is here.

If you're looking for a more open alternative to MMS, you can use the Shoutcast streaming protocol, which is more openly based on HTTP. Apache can serve as such a server, or something very similar, with mod_perl and the module Apache::MP3. That way, you get the best of both worlds.