As far as I can tell, the BLE radio in Coin is only used for detecting location, and doesn't expose any card details in any form.

Yea but it's still a wireless radio. Who knows what sort of exploit some malicious and clever person could come up with.

IMO, if we worry about that we would never use NFC or any other technology, really. I dare to say that everything is exploitable.

No, we should just always be careful with new technology.

If that were the case, how would the data input into your cell phone get stored on the card? I believe at a minimum Coin uses BLE to transmit card data from your phone to the device.