You would be testing everything as the engine will see it. Not just slew rate. Don't get me wrong I'm not saying testing on the distributor machine isn't a valuable way to test the system but it may not the same as with the components in the engine that drive the distributor or the actual power that is supplied from the vehicle to the ignition system. It's sort of like testing the engine on the dyno ( a valuable tool) and then putting the engine in the car and testing it at the track. I guess it could be argued to do both.