I guess, but the fact remains that using RFC1918 is still both more effort and less secure than just using a firewall is, and that NAT doesn't help the situatio

I guess, but the fact remains that using RFC1918 is still both more effort and less secure than just using a firewall is, and that NAT doesn't help the situation. Do you really want the security of your network to rely on your ISP always doing the right thing for you?