As a person who prides themselves on being frugal, I hate it when I miss a discount because I don't have the right card.
I'm always switching purses or wallets or fumbling and dropping cards on the floor trying to get to the right one. By punching holes in the corner and keeping them on a key ring, I've been able to make sure I have the right card when I need it.
Sometimes I'll put the clamp on the end and attach it to my keys if I'm not carrying a purse. This way I also always have those annoying Macy's or Victoria's Secret coupons that get sent every month. (keeping the post office in business though)