A game I'm not naming that is currently being often referred to as particularly good, happens to have a demo, and it turns out I can't stand it on any level (the story, the writing, the mechanics, the interface, the pacing, the balance). In other words, demos are a good thing and having one for your game is a good pro-consumer policy, even if it's going to cost you some sales in the short term (in the long term people who do buy your game will be more satisfied on average).