the notion of something useful being possible with an arbitrary
'video/foo' type that my application doesn't understand just doesn't
fly.
yes, but it does seem fairly common to have a "universal video tool" that
understands most video formats - perhaps even formats that the MIME
reader doesn't know about. having such a tool as the default handler
for video/* allows your MIME reader to play video clips even when the
specific video/ type isn't registered with it. similarly for audio/*.
presumably such tools would work with XML-based video and audio formats
also, if such ever became popular.