Rendezvous across a firewall is easy in the absence of 1:N IPv4 NAT. You just do an exchange like NAT traversal but it always works. Administratively opening a port is also easy.
Is it? Okay, so you don't have to specify the external-to-internal port mapping (and routers might offer you a default of external port = internal port anyway), but other than that it still seems like more or less the same procedure to me.
And at least my own home router does indeed use the same configuration interface for both IPv4 and IPv6.
If you have more than one device which needs similar external access, you won't have to use a non-standard port.
I have three devices at home which I'd like SSH access to. Because I only have IPv4, two of them aren't on port 22, and sometimes the non-default ports I've chosen are blocked by hotel Wifi etc.
Similarly, I'd like two different webservers to be available, but I can only have one on my single port 80.
For the more typical user, it can mean only one person playing a particular game etc.
Yes, that is of course a benefit, but what I was trying to get at was that you'd still have to dig around within the router config, so it'd still not quite be the radically easy, absolutely hassle-free peer-to-peer communication world.
Of course, firewalls are usually configured to prevent that. By default, on a home router.
But changing roles in the firewall is simpler without the need for port forwarding.