Why does it matter if a single media codec is listed?

[chrisn]

Why does it matter if a single media codec is listed? The restriction does not seem necessary given that the next bullet point gives freedom to the user agent to reject the mime type if it does not support it.
Note that there are conversations in MPEG about describing a single, layered video stream stored in a single track using multiple values in the codecs parameter.

Originally posted by @cconcolato in #222 (comment)