Add in a driver's license, three more credit cards to take advantage of various bonus point offers which depend on what and where you buy things, a corporate credit card, corporate access key car, and three loyalty cards for the supermarkets where you shop most often. Now imagine you have a birthday party and your six of your friends give you gift cards to various stores and restaurants.
I've been minimizing my wallet, but still plenty of cardboard buy-5-get-1-free coffee cards somehow make their way into it. It really depends on the individual. In any case, I think that the idea of this one-card-to-rule-them all is wonderful. I realize it is wishful thinking, but I want to see them pull this off.