Definitely not implying that two services should mutually depend on each other. But a third service C may want to look up in B for every record available from A - say, if B reports reservations, and A reports users who are members of a certain group, and you want the reservations relevant to a specific group. The OP article describes all sorts of pitfalls for the C team if you only had access to B via a "get by IDs" API.