That's still a stupid reason.

There is a MIME type for emails, so you can attach emails to emails, retaining their identity as emails, so the receiving MUA can display the emails you receive as an attachment just like any other emails, including the user interface for replying to them, or whatever.

Also, there is threading information in email headers. So, it is actually trivial to just gather together all the emails belonging to a specific conversation and attaching them all to an email to a third party that needs the history.

Using this bottom full quote nonsense is about as sensible as attaching a screen shot of a word document or something.