Plastic bank cards have some security weaknesses, they can be cloned or stolen. However if you're sensible they are a pretty robust way of completing transactions. By that I mean that you aren't likely to lock yourself out of your plastic card because you dropped it or more likely, ran out of battery.
The push to replace physical cards with smartphones, led by Apple and closely followed by Samsung, relies on two fundamental assumptions: replacing your cards with software means you need no longer carry your wallet and using your fingerprint to authorize transactions leads to improved security.
On the second count phone based transactions are way ahead of the game.
On the first however there's no headway being made. Your iPhone is an incredibly fragile thing, but it's the battery which really prevents it replacing your wallet today.
Knowing how tight iPhone battery life is, would you run the risk of being left with no credit cards at the end of a busy day? Yes, you may have never experienced the sickening feeling as your phone shuts down when you're unable to plug it in and still have a large chunk of day remaining. We were all smug until the first time it happened. It's awkward and inconvenient, but if your phone is also your credit card it could be much, much worse.
It's one of the reasons why Apple's continued pursuit of thin makes no sense. Adding an extra 1mm to the thickness of the iPhone would have no discernible impact on the phone's feel on the hand, yet could improve battery life dramatically.
I hope its something that all OEMs consider in the design of their next, because a mobile wallet that doesn't allow you to safely jettison your real wallet is no wallet at all.