Content-ID: could be used when one body part would like to refer to
another one. Imagine if you will that you had something like richtext
or son-of-richtext; you could refer to another body part in a
directive which could be used to indicate "display this graphic image
*here*, imbedded within the text."
Sure, nothing does this now, but it is another application very much
different from Marshall's use for managing body-part caches. The
potential usefulness of being able to name or refer to a particular
body part, and the fact that it is optional seems to argue against
removing it.
Louis Mamakos