Not all the time you are going to keep the card. Most of the time I give the card away as a souvenir. I would rather have them keep the card and remember about "that cool guy with the card tricks" and tell others and show them the card.
I don't know how many decks I go threw not too many. Because I am sort of poor you see. Don't have much money for cards. That is why I am trying to learn coin magic.... It is hard for me for some reason!
Interesting thread btw!