Well, there's plenty of people that subscribe on twitch.tv, but the livestream content is already free for all to see on twitch, subscriptions can just add access to chat feautures, emoticons, no ads, and VoD access, depending on what the content creator sets up.
I suspect that YouTube channels that choose to use this feature would do things like, early-release of videos on a sub-channel, or perhaps extended cuts and the odd special feature, but the majority of their content would still appear on their main/original channel.