Whatever the rule says, the practical reality is that Delta is not going to close your account because you've used one of the mileage summary services. This "rule" is just a way of encouraging you not to publish your pin in the Wall Street Journal, etc. Its like the rule that your bank might have that only you can use your ATM card or something. If you let your spouse use your ATM card, nothing is going to happen, unless you later make a claim the card was stolen, etc.