Some people use cards other than the KTTW because, as tstobb pointed out, the card that locks the safe is the only one that can open it. Each lock/unlock cycle is a new one. For example, I can lock the safe using my KTTW, but my huband can't open it with his KTTW. I would have to use mine. After I open it again, he could use his to lock the safe, but his would have to be used to open it again that time.
The reason some people use a card other than the KTTW card is because of this. If I was watching a movie and DH went to play bingo, we would each have our KTTW cards with us. If one of us wanted in the safe for whatever reason, we'd have to track the other down. Instead, some people use another card with a magnetic strip and then hide it somewhere in the room. So, whoever is in the room at any given time can get into the safe.
You don't need to use a credit card for this. Any card with a magnetic strip will do. My gym membership card, for example. Some library cards, etc.
As for losing a card and having to open the safe, Guest Services can open the safe no matter what card was used to close it, KTTW, library, gym, credit card, etc. Losing your KTTW card and having it replaced doesn't give you the same magnetic code back. It will be a new one so that your lost KTTW card will no longer be valid.
hope this helps,
Julie