Designers Tom Seymour and David Bruno said "With the Oyster Card Wallet you’ll know how much credit you have on your card at all times, at a glance. So no more waiting for the bus only to be turned away, and no more walking into the Tube gates and holding everyone else up. It's a simple idea that will help the London Underground work a little more efficiently."
It is a remarkably simple concept and I'd be interested to see if it makes it into production. Although as some people on Twitter have said, it might make it easier for pickpockets to tell which cards are worth stealing. That's assuming the pickpockets have great eyesight! Perhaps putting the display on an inside flap in the holder would help - although it wouldn't look as cool.