I lived in Gauteng up until about 2 months ago where it is common practice to drive without plates. I absolutely believe that you should report it (based on what I saw happen there).
IMHO a car should be declared unroadworthy and taken off the road if it doesn't have plates.
No person should be presented the opportunity to commit crime and get away with it without repercussion. It starts with speeding and escalates to driving over red robots and killing pedestrians.
Allowing citizens to govern themselves just doesn't work, everyone knows it.
For the nay-sayers: If one of your family members were killed by a driver without plates you would change your tune in a second. And you don't need to be a pedestrian to be killed that way. Speeders have much higher chances of surviving an accident than the vehicle they collided with.
Protecting and policing is the responsibility of every citizen. If you stand idly by while criminals take advantage of others then you are an incredibly low human, silently condoning their behaviour. Why should anyone help you if your family is in danger?
That is the problem with SA, people have lost self respect even flaming someone for doing the right thing. WTF.