Pirating the game for any reason is stealing the property - period. Any loss of sale is loss revenue - if the product is on your PC, and you did not pay for it, they have lost the revenue.
How did they lose revenue if I wouldn't have purchased the game anyway? They didn't lose a sale and they didn't lose the code.
stealing the product for any reason is wrong and lame
Good, logical and completely valid reasons to pirate games:
1) Many games don't have demos. Mainly open-ended games with RPG aspects. Oblivion, Morrowind, NWN2, Boiling Point, etc.
2) Many demos are a poor representation of the final product. Poor choice of levels (Hitman: Blood Money), incomplete assets, lack of polish, etc. Some demo levels are merely amalgamations of other levels (DX:IW, DMOMM).
3) Reviews are unreliable. Blizzard's games have all received critical acclaim but I enjoy none of them. Reviewers don't always review final code, either. This is why they manage to overlook glaring flaws that are so obvious to everyone else (such the incomplete nature of KotoR2). Finally, multiplayer games cannot be reviewed competently. They require months of playing before you can appreciate their depth (the reviewers of the Tribes games didn't even know how to ski).
4) The best way to judge a game is to play it in its complete, final form.
So there you have it. Good reasons to pirate games.