Well, they want to keep the response up front, while keeping the original statement intact, especially in cases where going through various mail gateways might mung any quoting. It is usually a short customer service real reply, or boilerplate in reponse to a long rant or detailed question, without having the sender reread the long bit they have already written. So in that situation, maybe yeah, it sort of makes sense, and I can understand why it might be corporate policy to set up mail that way.

But once that set up finds itself in less formal situations, such as a mail list, Usenet, Web Forum, Blog Comment, where reading in chronological order makes the most sense, it is a pain. Usually that 'default' top posting setting, shipped with many Microsoft products by the way, only raises its head in Usenet or Mail, due to the tool being used to access same.