I am one who brings ID, credit card, and a small amount of cash
We've had two situations where we needed payment. 1) We checked in went to Epcot, upgraded our tickets to APs...well, we were staying at a Value, so we had hit our charge limit when upgrading everyone to APs, so we couldn't charge to the room.
2) Another trip. We checked into Polynesian (this was concierge so someone helped us). Went to Epcot to buy a drink at a stand, MB was denied (and said something embarrassing like Over the Limit or something). I did have a backup way to pay. Turns out, the person that checked us in did something wrong. If all I had was my MB, then we would have had to go back to Poly to fix the situation.