Technically, when you charge with your MB you are not charging to that card on file, you are charging to your resort folio, and they are charging to the card that they have on file. Now, if you have a card on file in MDE, they will ask if that's the one you want to use, and it can all be seamless. Also, the card in MDE is there to guarantee ADRs and such. As mentioned, a PIN is set up to prevent unauthorized charging.