In general, I think piracy is full opportunities for big and small developers, but chances are if you have an opinion on this matter you'll already know about this and either reject or agree with it. I also agree with Gabe Newell and others that piracy is a wrong term because it doesn't take away, it "just" duplicates. The actual effects are happening regardless of the definition of the action of course.
There are many good reasons why to pirate from a consumer point of view, all of which are cheating the publisher of course.
First, DRM and other copy protection mechanisms are well capable of making your gaming experience hard. Maybe it's because your internet connection for verification reasons isn't stable and it blows you out of the game, because the disk is read every now for similar reasons and then your framerate suddenly drops because of it - again and again, etc, etc. All I know is that my pirated copy BioShock doesn't have sudden FPS drops and the copy I bought does (same system, patch, etc).
Second, people do not trust trailers, demos or reviews (anymore?). Maybe the 0.5h demo of the first chapter looks great but in the real game your only get 6 more hours playtime and things get repetitive fast. Maybe the trailer looked beyond beautiful, but parts of that scenery where dummied out or you can't play the game with settings that high. And reviews are very, very subjective in my opinion.
Third, there's no other "good" way to get the game. Maybe the game is 18+ like BioShock and you're 16 or 17 and your parents are actually keeping an eye on what you buy and don't like that huge guy with the big drill no matter how much you tell them about Irrational Games' history or BioShock's story-telling techniques that amaze you. Maybe you or your family simply doesn't have the money for new games all the time but you don't want to miss out social interaction or give up what brings joy and fun to your life. Maybe import is too expensive, but the localization in your country sucks and/or the game is censored but you still have to pay the full price for it, no matter how stupid and immersion-breaking neon-green blood looks like. Like I said, those are all things the regular customer has to put up with, so bypassing it via piracy is essentially cheating the company in that regard.
Fourth, with bought copies you rarely feel like you actually own anything. You only got the license to consume this product and the developers say how and when and not. You can't just copy a game or lend it to a friend. Or do you remember the times when you could just bring any game over to your friend and play it together? Some developers try everything they can to put an end to that. Not every copy protection mechanism is capable of doing that, but control of the consumer actions definitely goes hand in hand with stopping piracy.
I am convinced, and that apparently is also Gabe Newell's point of view despite using Steam (I keep mentioning him because he's head of a very successful gaming company, so that ought to give his voice a greater importance than mine), that people are very willing to spend money on things if the product convinced them, if it makes them actually WANT to support the developers.
For example, the Orange Box was a great package and especially Team Fortress 2 has received so much free support and content other companies *cough* would charge for, that a frequent question is how or whether it's actually worth the effort and show in sales. Some parts of the TF2 community are so in love with their game they even say "Hey, know what? Don't bring out a TF3 to get more money, we'll gladly pay again for our game in order to support you".
I got no statistics, but on the Facepunch forum I proposed that idea I received almost only agreement.
But again, you can't generalize. Not all pirates are misunderstood, poor angels, not all pirates are wearing eyepatches and want to rip apart the industry because they're greedy like the devil himself. But it's not like research is done on that matter, most companies just want to see them gone.