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The spec has some (arguably already limited) guidance on a specific MediaStream consumer, the HTML Media elements. It should probably define more generally what it means to define a MediaStream consumer; in addition to Media elements and PeerConnection, there is already AudioContext.createMediaStreamSource(), and I can imagine that Media Source Extension should define at some point a SourceBuffer.appendMediaStream() method.
We have a paragraph [1] that briefly describes what a MediaStream consumer is. That paragraph is followed by a note that mentions that consumers need to handle tracks coming and going. Perhaps the content of that note should be the first bullet in a list of requirements that are put on consumers. [1] http://dev.w3.org/2011/webrtc/editor/getusermedia.html#dfn-consumer
FWIW, while I still think this will be needed, this is more of a mid term issue, so it would be fine to close this bug as "resolved later".
Following Dom's recommendation.