Idle Curiosity: Why have a composite port at all?
The answer is simple. First the GPU has the capability to do composite but not VGA (and I don't think it can do S-video either) since not everyone has HDMI, it's a good idea to bring it out.
More importantly, however, one of the goals is to spread the use of computers to underprivileged countries. One very interesting thing one discovers when traveling through such countries is the wide spread use of TVs. So while composite is become archaic in say the UK or US or Canada, it's still very popular in other 2nd or even 3rd world countries