I have a big pile of club cards that I don't carry with me because they would make my wallet about three times as fat as it is now. Sure, I don't need to carry all of the cards all the time, and have managed to only carry the most used ones, but I run into the situation often enough where one of the cards I need I have left behind. If you have a phone with a big screen, now you can carry all of your cards without taking up any extra space in your pocket.
If you scan the front and back of all your cards and save them as jpegs on your phone, the barcode can be scanned just fine from the screen. The less I have to carry, the happier I am, so I'm definitely going to have to try this.