The first yellow card to Bismark Du Plessis was clearly wrong; the second to the same player was clearly correct (as were the cards to Nonu and Read) according to the rules. The problem was that the rules state that a second yellow card automatically leads to a red, and so the original error was compounded and South Africa had to play most of the second half one man short, to the detriment of the game.
Most of the discussion in the main-stream media and social media since has focused on the error by Roman Poite in carding Du Plessis for what was a perfectly legal tackle. This misses the point. Yes, Poite made was in error*, but errors are inevitable. Rugby is played at a furious pace. Split second judgements are required from both players and referees and all of them are going to make mistakes. The rules need to be written with a view that this is going to happen. The two-yellow-equals-a-red rule is simply too draconian to a world where errors of judgement can happen.
Part of the problem, here, is that there isn't any coherence in the incentives that the rules seek to create. Partly we want to punish individual players for behaving in a reckless way causing unnecessary endangerment to other players. Partly we want to punish teams for illegal actions of individuals that give their team an advantage. For the latter, it is appropriate that the punishment lead to an advantage for the other team in the course of the game being played. For the former, the punishment can occur after the game in the form of suspensions, fines, etc. If the point of a card is to put a team at a disadvantage to mitigate the advantage caused by some illegal act, why does it make a difference if the same player transgresses twice, or two players from the same team transgress once each? And if the point of the two-yellows-equals-a-red rule is to increase the punishment for habitual offenders, why does it make a difference if that player earns a yellow card once in two successive games or two yellows in a single game?
I come back to the rule I suggested in my 2010 post: If foul play merits sending a player off, let him be replaced so the game continues to be 15 on 15, but take appropriate action at the post-match judiciary (including being open to the possibility that the on-field decision by the referee was a mistake). If the problem is professional fouls, change the incentives so that conceding a penalty does not give the infringing team an advantage in terms of possession and field possession, and instruct referees to be more liberal in awarding penalty tries. But please, no more 15-on-14; it is a blight on the game.
* As an aside, why has the criticism all been placed on Poite rather than, George Ayoub. Poite made a call based on what he saw; he asked for guidance from Ayoub, the television match official. Ayoub instead of clearly stating that there was no foul play simply said that he couldn't make a determination and that Poite should go with the call as he saw it. Ayoub had the benefit of slow motion, multiple replays and different angles; Poite did not. Why, then, is Poite the one blamed?