I try to keep my daily carry limited to what will fit into my pockets, so I have a very small wallet. No space for all the different cards different places give you. For example, our library gives out a credit card sized customer card with a bar code. I never have it with me, and the librarians are usually happy enough to accept other ID. So far it has meant that I could not use their self-checkout, though. I wondered if the checkout machine would accept a photo of the card on my cell phone. Turns out it does! Wonder if this will work for my co-op card and any other bar coded items as well.
This is all good for convenience, but it also opens up a risk of fraud. Anyone who has taken a picture of my card can go borrow books with it. Or, anyone who has a picture of any number of gift cards that use the same system could get money off those cards. On a less nefarious note, it would also make it easier to share something like a gift card between multiple people, everyone would just need a photo of it.