It's funny that this is a suggestion because top-posting was a reaction to the unfriendliness of keeping message history in MIME attachments rather than the body of the message.
MIME may be the technically superior solution...But like Betamax and MiniDiscs, it lost out to the easier-to-use solution.
Message history wouldn't use that; it uses headers (metadata) like 'In-Reply-To:' and 'References:', which are in… well, I was going to say RFC 822, but it turns out they go back to RFC 724 from 1977.
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.
MIME may be the technically superior solution...But like Betamax and MiniDiscs, it lost out to the easier-to-use solution.