The prefix delegated to you does change in many consumer ISP setups, like the upthread poster reported and I've seen in many cases in well, I think repeating your assertion doesn't really advance your argument.
Are you thinking of a scenario where the ISP customers share a single prefix? This would be contrary to all the estabilished best practices and deployment guidance for IPv6 since it wouldn't let you easily subnet. And may get the ISP in trouble with your RIR since it's just extorting customers for access to v6 addresses.
