I think this essay has come to mind more often than any other in the past decade of my work as a developer, and sometimes distressingly, in life. From Tim Rogers “who killed videogames? (a ghost story)”
:

One click in one of these social games will take the user to the Real-World Money-Costing In-Game-Currency-Unit-Buying Shop. Here, the player will see that the game indeed offers him an option for paying $100 for something which is not real: an in-game currency with which to buy things in the game.

At the time he makes the conscious decision to wait for his energy to refill, the player likely already knows that “micro”-transactions exist which have $100 price-tags. Now he learns how much energy costs — usually, it’s nowhere near $100, or even $10.

Do players buy energy? What sorts of players buy energy? The short answer is: actual idiots. The long answer is: people who don’t understand why they have so much real-world money.

In social games, energy doesn’t exist to be bought. It exists as an engagement-regulating filter. The player attaches to it some vague notion of “value”. Backward-like, he comes to associate waiting an hour in the real world before coming back to the game with “working” and “earning” the “value” of the thing the game is giving him for “free”.

This isn’t exactly a truthful impression. The impression the player should take away — and gets confused about — is that in social games, time is a currency. Time is what you use to buy energy. Energy is a currency for purchasing in-game money, and some less-abstract in-game currencies (the premium in-game currency which the player must use real money to purchase) and more-abstract in-game currencies (namely virality and chance) can be used to purchase energy directly.

Energy’s multiple conversion rates into multiple in-game currencies mystify the idea of time as a currency.

The old idiom “time is money” has many meanings, you see.

“Energy” is a money that literally directly represents time.

…It’s a compulsion trap.