I'm not sure mate...I think neither said wanted to lose the test match after 5 really hard fought days. That's the crux of it, and there was more to lose by risking it in a two match series, especially for the home side. They were probably hoping India would attack for the three wickets needed, and they could sneak some runs in the process. India were hoping they would attack more and throw away the wickets...neither side blinked in the end.
The Wanderers wicket has a reputation where teams lose wickets in a cluster. Even in that match... up untill Du Plesis wicket, i'm with you, SA should have been more aggressive, but it's also hard to say if they would have been successful...AB and du Plesis wickets were directly a result of them trying to up the tempo..
I'm not sure if Steyn and Philander could have poked around for runs and still be wicket-less. There is a reason why batsmen leave balls instead of playing all the balls. There were about three or four singles that were not taken, but other than that they would have had to take some risks to score. That is why i guess they took it to the last over without losing any more wickets.