Multi-factor authentication is built to protect against remote attacks, which are the majority of fraudulent attacks. If a cell phone/tablet is lost or stolen, you should do exactly what you do today: call the carrier to report it and cancel service to the phone/tablet or do a remote lock or data wipe via another device. There is more information (contacts, email, other apps, etc.) on your phone/tablet that should be protected than one particular banking app. Keep in mind, that even if a phone/tablet is stolen or lost, the thief still needs to get past the username and password, hence multi-factor authentication. Similar to when you lose your ATM card, then the fraudster must know your ATM PIN to move funds out.