I think that seeing other people using the spells and hearing people talk about them is enough to make people want to reach mp6 so they can use them, Thus keeping more people active. If you let people preview the spells before they are supposed to be able to get them they will be annoyed when the preview is over and already have explored that aspect of the game so there is less to discover later on making it more likely for them to get bored with the game. I think people should have to wait.