"From" lines and "Reply-to" and whatever are headers that are meant to
be processed by computers. So, you can say all you want about how
dumb MUAs do or do not process these (and how intermediate mail
servers should keep their mits off). Now, humans use these lines,
too. So, call them dual use.
The subject line, on the other hand, is just for people.
Book titles are for people, too. Does that mean that it's okay for a
bookseller or library to change the titles on books, in order to help
the consumer indentify where they came from?
I'm a bit surprised at the frequency at which people who claim to be
networking protocol engineers fail to appreciate the benefits of clean
separation-of-function and layering.