> For most messaging in larger organisations, having "paper trail"-style replies that you can forward to others has its advantages

That brings up another point about using email for group conversations. If organizations hosted their own NNTP servers and commonly installed email clients also supported communication via NNTP, then this wouldn't be a problem. All messages already posted to a group would be visible to anyone who joins it at a later time. That's not the case with an email discussion that gets Cc'd to someone at a later time (since they don't have any of the earlier messages in their mailbox).