I suppose this whole bong sao argument depends on your definition of bong sao. As I see it, the actual bong sau part is the rolling of the elbow forward, the forearm angled slightly in front of and below the elbow. Attacks can continue from the bong sao position, and while some say the attack of striking forward with the elbow is part of the bong sao, I would consider it a different movement. Attacking with the elbow or an forearm can both be possible depending on the situation, but I don't consider either of these to be part of the bong sao.
The reason bong sao is performed so many times in the cham kiu form is because it is relatively difficult to perform properly, and needs to be slowly and continually trained like the fuk and gan in the siu nim tao. The cham kiu also demonstrates 4 variations of the bong, both as a soft movement as a result of redirecting movement, and also as a hard block for when the arms are down and not ready.