Jeff,
There is a simple answer to your question. This field was
intended for use precisely when the originator was confident that all
the recipients already had his certificate and the originator wanted a
very terse means of uniquely identifying it. One might argue that
this is an inappropriate optimization and that we should not offer it,
but that was the intent. I'm surprized that 1421 doesn't make that
point clear. if so, maybe we should revise 1421 to emphasize the intent,
ot reomve the field if everyone thinks it is a dangerous optimazation.
Steve