In fairness to Marriott, I'll add this:
(1) I always login to Marriott.com and check to see that the hotel is not actually sold out before I ask for a late checkout. (Yes, I know they can oversell hotels, but most of the time I'm doing this on either a Sunday or a Friday and it's pretty obvious that the hotel is not oversold.)
(2) The end result is typically good: I hardly ever get fully denied the late checkout. I usually talk to the manager, ask for what I need plus an hour, and end up getting what I want "as an exception for you, this one time only, even though I'm not supposed to do this."