The example they gave with the album during the presentation included a "as well as transferring the songs to your phone", which can't happen via NFC alone. Distributing powered back covers with Bluetooth would be ridiculous so I can safely deduct that there are physical contacts. The flash example is another proof.

Both BB10 phones as well as 808 have physical contacts to the cover if only to enable NFC so mechanically adding 5 more contacts to have full usb shouldn't be an issue.