The real question is: is there a need for MIME objects that can reference one
another? I think there is. An application/html document is a single "page",
yet you usually peruse them in sets, and mailing a set of such documents
around seems like a natural thing to do. (Mac users mail around hypercard,
stacks, don't they?)
I agree that such a need will soon exist if it doesn't exist already. I'm less
confident of multipart/related's ability to fill the bill when such an
application does arise. It is tough to anticipate what the future holds in any
case, but it is really hard when you're working in advance of *any* deployed
application.
At the same time, I don't want mime mail readers to incur the overhead of
copying components of a multipart/references set to separate files, if it's
not needed for that application.
We can always have both header-set as well as related. Alternately, we could
put a another parameter on header-set to say whether or not the parts reference
each other. On the other hand, it seems to me that the subsubtype is enough to
tell us whether or not the parts are related, so maybe we should just subsume
the semantics of related into header-set.
Ned