A) you want me to type my pin into a compromised device, the pinpad at the pump
B) you expect me to remember seven pins, I carry seven credit cards (yes that's excessive) and each should have a separate pin for security, right?
Ex of a simple and secure system. cc shows transaction cost, user clicks ok on the card. Card digitally signs a transaction with time stamp, vender ID, and amount.
Want safe online transactions, add a USB dongle or Bluetooth.
But it won't save you from a compromised point-of-sale system that lies to you about how much you're paying or which commits fraudulent transactions while the card is still in the reader.
Now, if only we carried around a device that included a display and some sort of input mechanism, plus a near-distance communication chip...
(Ok, if the device is a general computing device, a special secure operation mode might be needed for this sort of use case, one which can't be subverted by normally installed software, but still...)
Now people will complain that "the app doesn't run on their rooted, bootloader unlocked, jailbroken phones"
I mean, you can mod the brakes on your car if you really want to, at your own risk. What is a bit strange is when your media player can affect your brakes without you even noticing. Same principle here, less lives on the line.