Well, I was thinking about the same thing awhile ago, and I've come to the conclusion it simply wouldn't work. Let's say membership is $7 a month. You could easily speed through episodes 1-3 in that time, and instead of getting $30, the Aetherlight would only get $7. That is a big difference, and one the Aetherlight can't afford.
Another thing: to implement that system would take a lot of changes to the way the game functions. For instance, what happens if your membership expires while you still have quests? Would the quests disappear? What about rooms and clothes you've unlocked? Do they all disappear, or do you get to keep access to them? Do you get to finish the quests you're on and then no more show up? It would be like a whole different game set-up.
Alright, this may be a successful way of making money, but it's very frustrating for the player. I'd more likely give up playing the Aetherlight than spend money in the In-App-Purchase fashion.