I think a little clarification may be useful here, although I should add that I have no experience with this particular route (connecting international-domestic in JNB.)

Although it's true that bags cannot be checked through in many such cases it is often possible to tag the bags through to the final destination, and this is worth doing. For instance, when travelling from the UK to the USA or Canada with a domestic connection, I have to clear customs at the port of entry to the USA or Canada, so I must retrieve my bags to do so. But, if they are tagged to the final destination the subsequent drop-off is a very rapid affair which can be handy if the connection is tight.