I don't think anyone is saying don't use speed traps. Even in Germany the autobahn isn't entirely unlimited. In many circumstances speed is dangerous and in those circumstances a speed trap could be used to help reduce the danger.
I do believe that speed traps in South Africa are not really about road safety though. IMO South African roads could be made far safer by removing unroadworthy vehicles and punishing those who drive recklessly (taxis anyone?).
Police tend to trap where it actually isn't dangerous to speed, for example on long straights where they can pick up big fines. I don't often see them trapping where it would be truly dangerous to speed, tight blind bends for example. I imagine the reason for this is less revenue would be generated as people won't speed in the truly dangerous areas unless they are maniacs. Anecdotal, but I would more than welcome a formal study.
I got a ticket for not obeying a stop sign the other day. There was no one else on the road and while it is true that I didn't "stop" in that my speed was not reduced to 0km/h, I did slow down to around 5 or 10km/h before the stop street and make sure that the other roadways were clear, there were no pedestrians etc. there was also plenty of visibility. So... where was the danger?
EDIT: incidentally does any know if our police have a policy of trapping more in high-accident zones than elsewhere like they do/did in the UK?