While it's nice to offer promotions, how many free night certificates go unredeemed? You stay your nights, qualify for a free category 1-5 free night, then do you actually use them? For me, these stays are accumulated on business travel. Staying at a Courtyard or Springhill is out of convenience. When I'm on vacation with my family, I'm not staying near an airport. I want to stay at a full service Marriott or vacation villa. Guess what? I can't use my free night certificate for any of these becaue they are category 7 or higher. There is no process to combine multiple free night certificates for a higher category hotel. There is a one time exception to convert them into points. I'm guessing the majority go unused and Marriott is not out anything. However, they gave us a nice promotion!
Reward your members by actually giving them a promotion they can use!