Out of curiosity, how many do you have? I think 8 would cover all of the cards I use at least once every 4 months.
Even though Coin doesn't solve this problem, especially since it doesn't have EMV which is required in Europe, I'd love to have all off them one card. I have another boatload of cards at home that are rarely / never used because I never have them on me when I could use them.
So basically: very cool idea, needs some more thought for the european market though.