JOIN/PARTS/NAMES cause a heavy burden on our servers, particularly for large channels (currently defined as >= 5000 users). To protect chat from becoming unstable, we decided to limit these for large channels. There is an API: http://tmi.twitch.tv/group/user/{channel}/chatters but make sure you handle errors appropriately, as its not the most reliable API.
We are aware of this issue, and this is one of the issues we want to address going forward. Knowing when people join and leave a room is very important, and we want to make sure that we support that feature for all channels. In the short-term, we decided having people connect and send messages via chat was more important than keeping this feature fully-supported.