Exactly this! Especially when you have multiple internet feeds into your router, but no assigned IPv6 block. Eg Router has fibre from one carrier, and LTE from another carrier, and you either want to actively use both in a load balancing fashion, or use them in a HA/failover fashion without paying through the teeth to the ISPs for the privilege.
Fibre with LTE HA is an extremely common scenario in the physical retail world. And retail is notorious for tight margins. With IPv4 this is dead simple for a router to do using NAT. With IPv6 it’s a nightmare because everyone is against NAT in IPv6. It took ages to get some form of NAT, and it’s poorly supported.
Mostly agree, though to nitpick on a particular POV:
> multiple internet feeds .. assigned IPv6 block ... paying through the teeth to the ISPs for the privilege.
People like having their own network blocks, but typical route-my-subnet-here arrangement require all core routers to store the path to it. IMO assignments of globally re-routable addresses should be actively discouraged and you shouldn't get one unless you're an ISP. Not defending current payment arrangements.
Roaming should be done at or close to the endpoints - imagine if all internet routers had to keep and synchronize ~12G entries, usually in special CAM memory (currently at ~800K) - that does not scale, at all.
Fibre with LTE HA is an extremely common scenario in the physical retail world. And retail is notorious for tight margins. With IPv4 this is dead simple for a router to do using NAT. With IPv6 it’s a nightmare because everyone is against NAT in IPv6. It took ages to get some form of NAT, and it’s poorly supported.