I needed to book LHR-CDG last month in Y (full fare) and my TA was told that the flight was completely full. I reminded my TA that I was a Gold member (the TA still had me in their records as a blue) and within 30 minutes my reservation was confirmed through reservation assurance. I was told in the lounge that the flight had 10 on the waitlist (possibly one of which was the poor person I had bumped off!) The flight was, needless to say, packed. So in my experience it does work as long as you remind them of their own rules. Also have never failed to clear a waitlist and this is one of the key benefits for me.