Keep in mind this perk doesn't work on weekend nights or holidays, too.
If there's an absolute "must-do" for you that's last minute and it's a weekend night where the room key won't do you any good, try calling the restaurant direct.
I had a situation our last trip a little over a month ago on a Friday night. We were hoping to head to NBA City for dinner. I called dining reservations from the HRH hotel phone and was told I wouldn't need them, to just "flash my room key." A while later, I was bored (waiting for the rest of my group) so I read that book that's in all the rooms. There was a disclaimer about the room key not acting as a next-available seating on weekends and holidays. So I called back to dining number to see if we could just make an actual reservation. They told me they couldn't do that; that they could transfer me to one of the dining kiosks. I was on hold forever until I finally just got disconnected.
I called 411 on my cell phone, got the number for the dern restaurant and just called them direct. The lady on the phone from NBA confirmed that, NO, your room key will not get you priority seating on weekends, holidays or SPECIAL EVENTS (like a bunch of basketball games on, lol), but you can absolutely just call the restaurant direct and make a reservation.
From now on, every time I go to USO, I'm just going to bring the restaurants' phone numbers.