Re: "What's wrong with a /64 prefix?"
SLAAC on two networks, e.g. main plus guest or IoT networks. You don't need any more justification than that.
/56 isn't a big allocation. In terms of fraction of the total available space, it's 1/256th of a single TCP port of a single v4 IP. Nobody should need to deal with an allocation size smaller than that, and the question of whether a /64 is sufficient or not should never need to be asked by anybody.