To return to not making your content fun, I believe MMO content should be designed on a scale. On one end you have rewards, and on the other end you have fun. The more fun said content, the less rewarding it should be, while the less fun something is, the more rewarding it needs to be to stay viable/relevant.
— SynCaine, Fun vs Reward
In terms of designing a subscription-based game that retains players and makes efficient use of content, it makes a kind of sense to design a Skinner Box that rewards your players for spending time on the least fun content. This is where Moloch drives MMOs.
Pay to play a game designed to minimize fun per hour. The main thing we seem to learn from playing MMOs is that you will have more fun not playing MMOs.