McDonald's sees a spike in their sales go up every time they employ their strategy of affixing Monopoly pieces to their soft drinks and fries and a few other choice products. They reward those who super size their meals with double the Monopoly pieces. The game is so popular that if you wanted to increase the sale of a less popular item, such as the filet of fish, you could simply apply more game pieces to its wrapper.
The same phenomena exists with a proof of purchase system. Everyone remembers Kool-Aid points from their childhood. You could purchase a number of Kool-Aid branded memorabilia with a large enough number of points. Pepsi and Coke used a similar method. Pepsi even made the mistake of advertising that with 35 million Pepsi points, you could purchase a harrier jet. Taking this literally, one man with a corporate backer attempted to take advantage, only to be told that this was impossible and the company became embroiled in litigation surrounding this promotion. People were more prone to purchasing Pepsi instead of Coke during the period when Coke did not employ a similar system.
The reason I'm posting this is that gimmicks don't always work, but the concept of scarcity suggests that people's wants are infinite and are prone to consumerism. If you tell some people they want it, it just may work. It's like a Jedi mind trick. You can reward people who purchase your product with items that do nothing more than advertise your product either in the home or in the form of apparel. It doesn't work all the time. It takes the right product, but with enough creativity, this concept of “incentive to purchase” can be a home run.