Top posting is extremely valuable to people who need to weigh in on parts of a discussion without participating in the entire thing from the beginning, like lawyers, accountants, engineers, and doctors, who may get added on late into the discussion but may need the message history so they can make informed statements/decisions.

Also, the suggested alternative, MIME attachments, simply doesn't work well in most mail readers. It's barely usable in Gmail and not at all usable on iPhone or Android. (Ironically, it works quite well in Outlook.)