On the other hand, a recipient does not reliably know whether a particular
address is subscribed to a list and whether that address wants duplicate
copies. So it's helpful if there's something that tells him this.
Quite.
And while we are thinking of "fanciful" cases :-) how about the case
where somebody sends a message to a list to which they are not
subscribed, but nevertheless does not want to receive any followups?
("You guys might like to discuss this; I don't have time / am not
interested / whatever.") I think that could be done with MFT, couldn't
it?
In the sense that the responder doesn't send a copy to the author of the
original message, yes.
In the sense that the responder _realizes_ he's not sending a copy to
the author of the original message, no. Because use of MFT for the
case where the author doesn't want to see the reply is indistinguishable
from the use of MFT where the author is subscribed to the list.
Keith