A standardized method of sending a set of email messages as messages, rather than copied into body text, predates Microsoft Outlook.
MIME may be the technically superior solution...But like Betamax and MiniDiscs, it lost out to the easier-to-use solution.
A MIME digest would be used for the case that you want to provide someone a bunch of messages they don't already have. In that case, this brings up another reason this is superior to including the thing as text: it preserves all the threading and other metadata.
Insert your cursor in their message, press enter and respond to every part worth responding to - inline. (you may want to sign your entry)
Remove all parts of the previous message(s) that do not directly build towards the current conversation.
It should look so organized that the new participant immediately adopts the format. If they fail, do it for them. (preserve the sane part of their signature)
Doing it like this really feels like you are talking with serious people who would never waste your time. That said, you can now ask how their weekend was. That part of the conversation is simply removed later on.