Actually the Rotunda/Super Star/4EAT test system would perform pretty intensive diagnostics. You would unplug the ECU under the dash, plug in the rotunda adapter, and all those sensors and switches connected to the adapter could be tested. It requires a big 'computer' and lots of hardware, financially out of the reach of home mechanics and non-Ford neighborhood mechanics. It seemed to be mainly used for static testing of circuits with the engine off, including automatic transmission testing.
Eventually someone figured out that it would be cool to have the smart diagnostic computer in the car itself, and OBD2 was born...