Perhaps that was the original goal, but now-a-days its pretty rare for a customer to turn off his or her modem, so its unlikely a customer would ever give up an ip address that could be used by someone else.

If pooling was the true original reason for dynamic IPs, it usefulness for that purpose has already passed, regardless of IPv6.

They'll do whatever is cheaper and then they'll tell you it's dynamic (or "not guaranteed to be static") as an incentive for some customers to upgrade to business service.

I could definitely see scenarios where you end up connecting to a different CMTS and thus it's cheaper for them to assign a new prefix rather than carry your old prefix in their IGP. Or something like that.