The difference being that somebody who pirated a game may turn around and buy it if they enjoyed it and they find an attractive price point for it.
That's not very likely though, the value proposition drops significantly when the consumer has finished with the product. It may happen but it's not something the industry can count on. I understand the whole demo replacement argument but it fails to take into account human nature unfortunately. What people say on the Internet and what they do are often two radically different things.