I haven't seen any
arguments that MFT is harmful
It is harmful because it results in an increase in spam; e.g.:
"Hi. This is the qmail-send program at cr.yp.to.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.
If an MFT spec were adopted, I can't imagine that it would recommend such
behavior. It might even discourage such behavior.
I think MFT is sub-optimal because it is difficult for ordinary users to
use, because it can be confusing for recipients, and because it has to
be widely deployed in recipients' MUAs before it becomes generally
That, and fundamentally, letting the sender of a message specify "reply
to author means x, reply all means y" for a recipient just seems wrong to me.
Reply to author means send a reply to the person who wrote the message.
Reply to all means send a reply to everyone who received that message.
If those buttons don't do something fairly close to what they say they do,
and if the UA doesn't make it clear when it's doing something slightly
different than what the recipient asked it to do, the UA is terribly broken.