(You might get a free CGNATed IPv4 as well, but the point of v6 really is not to have to.)
Even with both one might decide only to use the v6 uplink. There's not really (ordinarily) a good reason not to - you can still use v4 locally if it's more familiar/easier to firewall or whatever, and unlike a v4 that you haven't paid extra for, the v6 is almost certainly static - which is either a huge benefit, or you don't care either way.
(You might get a free CGNATed IPv4 as well, but the point of v6 really is not to have to.)
Even with both one might decide only to use the v6 uplink. There's not really (ordinarily) a good reason not to - you can still use v4 locally if it's more familiar/easier to firewall or whatever, and unlike a v4 that you haven't paid extra for, the v6 is almost certainly static - which is either a huge benefit, or you don't care either way.