Again, most uses of Passbook don't involve payment. They're used for the convenience of checking in or for offering coupons or reward cards.
Which reinforces my post above. Passbook is just a temporary "stopgap" that Apple is using for now, since Apple is not yet 100% confident about jumping the NFC bandwagon.
In the months (years?) ahead, when NFC begins to prove itself after a shakedown, Apple can easily just phase out Passbook... or roll its functionalities into Apple's own NFC front end.
Hence.... Apple probably never meant Passbook to be a "competitor" or see it as their own proprietary replacement for NFC (which is more far reaching and can do more). It's simply Apple saying "We don't think NFC is ready for prime time, but we keep looking into it and assessing when it might be ready to be implemented on Apple devices. In the meantime... we have this thing called Passbook!"