For me it is cool cool to be able to disable audio and video separately; *as a feature*.

The easiest way to do it, is being able to mute/unmute audio and video separately, while the purist way to accomplish it, is doing it through reINVITEs and media direction awareness.

If the first way doesn't break the design and becomes a first step to offer the feature, I find it ok. It can later be done by changing media direction through reINVITEs and SDP management, but that would be transparent for the user by the time the change is done.