It seems to me there are a two kinds of pirates: one that is legitimately trying to test out a game before buying, and one that will use any excuse to justify why they aren't paying for games.
For those of you who download a game and try it for a little while, I sympathize with you. There are some crappy, poorly made games that aren't worthy of the budget bin (any game made by Derek Smart, for example). If you download a game, play it a couple hours (or less) and then delete it (or run out and buy it). Fine, you've taken the equivalent of a test drive before you buy a new car. You either hated it and removed all association with it, or decided it was worth your money and supported it.
Then there's the other kind of pirate. They too justify their actions as a way to ensure they don't waste money on bad games; however this group needs a much longer test drive.
Let's take C&C 3 for example. Of course the demo isn't a good enough test run for you because, of course, a tutorial mission, 2 campaign missions and a skirmish mode aren't enough of a test drive to see if the game's worth picking up. You enjoy the demo enough to hunt down a torrent for the full version though, because you suspect it's worthy of your play time. As a result, you pirate C&C 3 and play through the entire GDI campaign and enjoy it. But wait! Maybe the Nod campaign is different! You have to try that too (wouldn't want to waste your money, after all). So you play through the Nod campaign and enjoy it as well, but what you really enjoy is a good multiplayer game, so you play every available multiplayer mode and have a lot of fun. However, the single player "was just too short" and multiplayer mode doesn't offer 25 different game types and doesn't support enough players, so it's just not worth your money (however you will keep it on your hard drive "just in case you change your mind"). Good thing you tried the game out before you bought it!
The bottom line for me is that if you "test" a game for more than a few hours, you should buy it if it's good enough for you to keep "testing." On a side note, I don't know where you people buy your games, because the vast majority of mine were bought for under $50. Supreme Commander, for example, I just bought last week at Circuit City for $38. It's certainly not uncommon for new PC games to go on sale at Best Buy, Circuit City, Frys, etc. for a good chunk less than $50.