Yes, sometimes it is worth a few hours' entertainment at most (like the Sims - I got bored of it fairly quickly, but all my friends seem crazy over it for some reason. Small wonder that they stereotype female gamers). Or sometimes it's total horse manure for which I'm glad to never have paid fully for it (I am not paying 200 HK$ for things like Diablo 2, sorry).
Oh, and ruining game's market? Ah, of course. One person doing it and the whole thing will come down around your ears. You know that some games simply don't have demos, or the demos are poor examples of what the game entails, right?
FYI -- my copy of DD is legitimate.