Just about any scenario where money is held as cash or a deposit in a bank account could apply equally to Bitcoin. You give a stack of $100 bills to a bank, they give you a slip of paper showing they have your money and will give it back when you ask. If the bank president or a crooked teller then puts everyone's stacks of $100 bills in a suitcase and disappears, your slip of paper isn't worth much.
In the case of the Pirateat40, it was a classic Ponzi scheme. The fact that it was done with Bitcoins instead of pieces of paper with pictures of dead Presidents and statesmen on them doesn't change much.
This would be a good analogy if you substituted "some guy in an alley" for "bank".