What is the downside? The external network only needs to know you're responsible by an IP range that your ISP gives you, you can still configure your network internally as you please.
The addresses depend on the external network. If my prefix changes with IPv6, suddenly nothing works on the internal network until all the nodes are reconfigured and the firewall rules rewritten. I know because it happens to me regularly.
With NAT the internal nodes can for the most part work like normal. They might experience a gap of a few seconds where communication with the external network (the Internet) is down, but apart from that nothing.
You might say just use ULA[1]! To which I will say what's the point of IPv6 then?
It's true that this is a problem for intermittently connected consumer networks and customers of flaky ISPs that change the prefix on you needlessly. Regulation for allocating PI space to flaky isp customers might help.
