One could summarize that if you want people who get the
forwarded message to respond to you, then use a forward
facility, including the incoming message in the body of
a new message. (For some reason, text inclusion is
most common, inclusing as a message/rfc822 body part
is not so common.)
This has not been my experience. I receive roughly the same number of MIME
encapsulated messages as I do text-encapsulated messages. Indeed, I routinely
receive mail that has been MIME forwarded multiple times, and I've seen
real-world cases of messages nested almost a thousand levels deep.
But both of these tend to be overused. In particular, they are often used
when resending would have been a better choice.
Ned