Public strokes are not as much about inflicting fear, as it is not that hardcore to get hit in your bottom 4 times.
They seem more like a tool to disgrace a person, shatter his dignity and undermine the authority of his position.
I bet that Saudi diplomat will remember that episode for the rest of his life very vividly.
I don't think that in Singapore, the caning is administered in the open. So the humiliation part is not that severe. However, in countries such as Malaysia, Indonesia and the GCC nations, it is given out in the open.