In a "large complex system" the subsystems might be so large and so many, that it's no longer feasible for the key implementors of each subsystem to get togethe

In a "large complex system" the subsystems might be so large and so many, that it's no longer feasible for the key implementors of each subsystem to get together and discuss technical details or participate in stakeholder and management meetings. They might also have difficulty ensuring that architectural constraints which are not directly visible to them are respected. So yes, a dedicated role is needed.