How would this work if your upstream router doesn't support v6? All other networks send packets towards you, but your ISP ignores them because they are invalid packets. Then what?
If you want to start using v6 before you're sure that every host supports it. But this isn't strictly needed, could just rely on something in DNS telling you the host supports it (I'm not saying AAAA records cause idk if you'd use them in this scheme).