The one time I needed to use this benefit was in 2011 when a Crowne Plaza I used to frequently stay at was fully booked.
When I called the hotel and politely requested the Guaranteed Platinum room availability the hotel indeed was able to offer me a room at the full rack rate.
I even got the Platinum upgrade, which I was not expecting considering they were at capacity, but the suite more than made up for the rack rate I paid for the regular room.