I strongly discourage anyone from using such cases. Every time you take out your phone, especially if you are picking up a call, your cards are revealed to bystanders in all its glory. In a way, you are flaunting your wealth, enticing people with ill-intentions to find ways to cause you trouble.
Its worth noting that keeping your valuables hidden, i.e. in a proper wallet in your pants/bag will not prevent you from getting pickpocket-ted entirely, but research has shown that people are more likely to commit acts of thievery when they can actually see the item itself*.
Just that day I saw a lady with a Samsung Galaxy Note, in a nice red, leather casing, the one that opens like a little book. On the inside of the left flap she had at least 4 cards. And she was just watching videos on her note while the flap swung around with the movements of the bus. I could almost hear the cards screaming out “HEY LOOK I AM HERE”.
And also, I’m not sure how secure these holders are, or the iPhone cases above are, haven’t got one, but chances are Murphy’s law will come into place and a card might just slip out as you pull your phones out of your skinny jeans or something. And look, it blocks the camera.
*Research by yours truly based on 0 test cases. But I’m right, trust me.