"So the temptation to keep the oil will always be exactly matched by the temptation to sell it."

Not necessarily. The flip side of ever higher price of oil is ever lower value of money, since money is just the implied promise to perform work, and work requires energy, which comes from oil. So, you can treat both oil and money as currencies, except one of them keeps its value. Which one of them, then, is a better currency to hold in reserve, the ever-shrinking dollar, or the perennially valuable barrel of crude?

A lot of these countries say they are building a non-oil economy out of their oil revenue stream.  Regardless of how real that commitment is (or its possibility of success), it also serves to silence critics of a fast pump-out.