It would be great when you have a really super fantastic upgrade card that you don't currently have the img for/want to wait on upgrading it if you could lock that card. The lock card would be like a reshuffle or get out of hell card. Regardless of how many you have you can only use one at a time. That way if the other 2 cards are things you don't wish to upgrade you can reshuffle without losing the one you want.