You've got to think that at least some of the people who download it would have otherwise purchased the game.
I can also see how the price of games could go up due to piracy. If the game doesn't make as much money as it should, what other ways can the company get it's money back? Every game released has to try and at least make the development costs back, otherwise gaming studios would all be bankrupt.
The computer game industry still isn't on a par with the movie and music industry in the financial department, and on occasions it hits them hard.
Also the less money a company potentially gets back, it could also mean less money spent on the game itself in the future. I don't know how much money is lost per game across the board, I'm sure EA, Nintendo and Sega have a better idea than we do, but without piracy, quite conceivably the computer game industry could be in a much stronger position.