>> There's almost no advantage at that point versus actually having a NAT.
Sure there is.
You avoid starving the router of TCP (or SCTP) ports. You avoid having to mangle TCP packets because they happen to contain addresses. You avoid IP address based "one-connection-per-client" limits on servers.
In short, you can use simpler servers and routers. Which translates to fewer bugs and less power-hungry CPUs.