Users would join the service if it had a game they wanted. Its games and Steam's games would not have much overlap, if any. Having it seperate would also allow

Users would join the service if it had a game they wanted. Its games and Steam's games would not have much overlap, if any. Having it seperate would also allow them to change the default UX. Among other things, the default behaviour of sending notifications to your friends like, "Bob has started playing Call of Duty," is probably not the right behaviour for porn games. If they don't firewall those sorts of games off into a seperate area, Steam at least needs to clearly communicate at every point where the user interacts with the game on Steam (store page, cart, profile, library, etc) whether that interaction is public or private, because historically just about every interaction has been visible to the user's friends.