I know guys that will probably pirate Skyrim for the PC, install it, play for roughly an hour if that, quit, and probably never go back to it. Yet the gaming industry says "OOH EEMM GEEE PIRACY IS KILLING DEV COMPANIES", I say bullshit. That same person was likely not interested in the game to start with, tried it out and didn't enjoy it. They never were a potential customer, and they havn't "stolen" the developer's hard earned money. Not to say that it's justified, but it's not much different from getting a demo, and don't get me started on the lack of demo's available on the PC, we used to receive demo's before games were shipped, now we get trailers...
I know they will often pirate the game, and don't end up playing it for more than 1-2 hours max, but often consider it when they pop up on steam as GOTY editions or at greatly reduced prices.
Like Lycanthrope says "pirates aren't customers", but potential customers may pirate a game to see what it's like. It's become far to convenient to pirate games, and you know what I'm not that surprised considering all the "day-1" patches that come out with a lot of today's games both PC and Console.
I say developers could try to embrace piracy as a free mean's of advertising, some actually have. Games are becoming more socially interactive, and I think it will be a requirement very soon just as Multiplayer is almost a requirement in modern games. All you need to do is simply turn off all the social elements to your game should someone pirate it. It almost forces people to want to buy it if all their friends are playing online and they are not included in that shared experience.
A good example of this would be Diablo 3, you can bet your grandmother's baking tray that people are going to pirate the shit out of Diablo 3, but the game is designed to be very social with friends dropping in and out of your games, sharing the experience together. So how many people dungeon crawling solo are going to want to buy Diablo 3 when all their mates are playing it? Yeah thought so... all of them.
As for people complaining about games requiring you to be online, well get used to it, we do live in the 21st century you know

Xbox and PS3 require you to be online to get the complete experience in alot of games, the PC is no different.