You're right. It DOESNT extend actual play time. But what it does do is keep the game on people's mind. Many would finish and grind out the game within a month and not play. But by time gating and keeping you guessing, you check the forums daily to make sure you aren't missing anything. You log on to do that next step of the quest. All of this gives them another opportunity to catch you and keep you a little while longer cause you know... Who wants to just log on for a quest step... If you took the time and effort, you'll probably run a patrol or run a strike or bs around. This makes the world seem more populated. They can also intentionally time gate things on major release dates. I bet sleeper stimulant will be back November 10th. That is the day ESO DLC comes out and Fallout 4. They are anticipating losing players, but if they make it so that you have to come back because of a new timed exotic, they can peak your interest long enough not to leave.