I think a source of much disagreement on these list has to do with the
differences in assumptions about the lifetime of SMTP- and 822-based
mail.
If you think that SMTP and 822 are short-lived, then only minor
adjustments, if any, are going to be practical. Furthermore, it will be
impractical to hunt down and get fixed all the implementations that are
misimplementing the protocol, to say nothing of getting any enhancements
(no matter how slight) implemented on all the mail relays.
On the other hand, if you think that SMTP and 822 may be slightly longer
lived, then both extensions to SMTP and 822 as well as hunting down and
fixing mailers which are unenhanced or misimplemented is just part of
network normal operation.
-David