The problem is that the addresses might not be where you need them. Let's say you bring an OpenVPN-running Linksys router to a hotel room and plug your devices into them so they can be on their own firewalled subnet with VPN access (this is the ultimate road-warrior setup). Unless the hotel routes a subnet to you (and they won't be), you can't do this. The hotel's subnet might have plenty of public IPv6 addresses available, but you won't have any to make your own subnet. What should you do instead? Bridge the networks and use layer 2 firewalling? The VPN routing would be a nightmare. This question has been plaguing me.

To be clear, I do not think NAT should be used in most cases. But for rare or temporary cases it would be very useful to have available if you know what you're doing.