You make some good points. However, we tend to be more based on a Westminster government structure, yet we have a president.
The problem with the current system is that the head of government is also the head of state. How often does Mbeki have to turn up and be accountable to parliament, a couple of times a year? In the UK the prime minister is there every week. He doesn't only have to deal with the opposition but his own party, because half of them were elected based on voter nominations and not party nominations and they're looking after their voters. Not too many countries share the SA system. Getting rid of a president who's both the head of government and the state looks really bad, but getting rid of just the head of government doesn't, that's just internal politics.