Cricket is all about balance. Fortunately it's not all it's not always about the best 6 batsmen and the best 4 bowlers. The match in S.A today is showing that while it's not critical, it's important to have that dependable 5th bowler, in this instance for the occurrence of injury. J.P didn't set the world on fire but he took a critical wicket and just as importantly tied up one end with economical bowling which kept things tight and allowed the front line bowlers to be rested and rotated efficiently to keep them as fresh as possible. Importantly this selection didn't weaken the normal batting lineup and he also scored a Hundred in the 1st innings. Additionally Du Plessis took two stunning catches in this match that helped to heavily swing the match in his teams favor, two catches that many lesser/other slippers may not have managed to hold on to.
When selecting a test XI one of the batsmen should be able to at least help out as a 5th bowler to either take critical wickets and be a partnership breaker or at least keep things quiet and the run rate low when they are operating. At least two of the players as well should be very good to great slip fielders who can take those game changing grabs and turn half chances into wickets. That's why players like the aforementioned Hammond, Sobers, Greg Chappell, Warne, Simpson, Kallis, McCabe, Barlow, Mitchell etc were so valuable as they brought both supplementary skills without a compromise to their primary responsibilities while contributing to teams wins and successes. Then they are the countless others who would have contributed in any one additional discipline while being uncompromising specialist/greats including batsmen, bowlers and keepers.