We already have "client" and "server" addresses, courtesy of NATs. NATs are a huge pain from an architectural point of view, but all the same, many people like them because they provide some measure of security as a side effect. That paper was all about trying to capture this within the architecture, and providing the same effect in depth, rather than just at the NAT itself. It was a thought experiment more than a serious proposal, but that's the way new ideas come about - someone proposes a radical-but-flawed idea, and others see some merit in the principle, but find ways to avoid the flaws. I wasn't trying to push this solution on you - just responding to the over-general per-flow state point.
Except that the stateful packet filter that sometimes comes with a NAT gateway (and that provides the security that people like) depends exactly not at all on NAT and the pain that comes with that kind of address class distinction.
Well I don't have stats but from personal experience (perhaps I have a bit more experience with french rather than US providers), home routers do not use a specific stateful firewall, and use masquerading NAT. So it would depend on the way NAT is done.
