Assume that you get an IPv6 address assigned to your router. Assume that a computer on your LAN wants to talk to a internet host with IPv6. The NAT box can translate replies from the internet host to IPv4. But how are you going to talk to the IPv6 host? How can you send a packet to an IPv6 address if all you got is IPv4 on your LAN?
I suppose the NAT box could run DNS and make a look-up table mapping IPv6 internet addresses to IPv4 for your home computer to use. This seems a bit of a kludge and it doesn't help you with raw IPv6 addresses.
Clearly, we are stuck with IPv4 for legacy devices for at least 10 years (estimate based on time for floppy to die after it became somewhat useless). Assuming IPv6 does come (I am not certain we won't be living with some awful kludge instead), you will want to also do IPv6 within your LAN.