dg1, the issue is not whether you have a confirmation number. The issue is walking up to the front desk and asking if there are rooms available and being given untruthful information.
You've been lucky though. It's really a good idea to have your confirmation number handy, and in my experience, especially at Marriotts. I know of 2 different people who've had guaranteed reservations at different Marriotts and when they arrived very late at night, their room had been given away and the hotel was full. The hotel found rooms for them at other hotels (both times, far less desirable) and Marriott paid for the first night.
If something similar were to happen to you and you did not have your confirmation number, you would have no proof of a reservation and you would be on your own to find a room elsewhere, at your expense.