Honestly, I think everyone here is too busy calling other people names to understand the fundamental issue.
Piracy is NOT the same as "stealing" or "theft" by most US laws. It IS, however, illegal. I in no way condone, approve of, or endorse piracy by making this distinction.
Piracy is generally accepted as immoral or at least amoral--as a pirate, you obtain something that normally costs money for no money beyond the expense of an internet connection.
The question lies in whether you would have bought it if legitimate outlets were your only option. I believe this is OFTEN (but not always) the case: people pirate "extras" that they would have not bought normally--in other words, no extra legitimate sales would have been made.
People no longer really have the "as a demo" excuse. There are simply too many gameplay videos and "Let's Play" videos on Youtube which show exactly what the game will be like. People who pirate as a "demo" are justifying their actions illogically (and before anyone says "regional locks on Youtube," I'll say VPNs exist, and you'd probably want/need one for pirating anyway). I'd say that any pirating TRULY as a demo, regardless of the little sense it makes in terms of risk (and not just as an excuse) before purchase doesn't result in any lost sales because the pirate goes on to a legitimate purchase or uninstalls it.
iThe most common excuses remaining are "not available," "can't afford," and DRM. In the first two scenarios, there is no or little harm done--if they don't have the ability to buy it or the money to spend (again, as a legitimate reason and not an excuse), GENERALLY no sale is lost. Some, of course, would have saved up for several paychecks to get a game legitimately. I cannot estimate at that number, but I suspect the intersection between "too poor to buy games" and "wealthy enough to have a gaming system plus VPN and uncapped data for piracy" is pretty small.
For the people who do it due to DRM: it honestly depends. Some people would refuse to buy a DRM game to "vote with their dollars" but then pirate because they really want to play it (after cracking the DRM, of course). Others will do what I did--legitimately buy a game (ACII, in my case) and then use pirating to get a cracked version or a workaround. In these two cases, no sale is lost. In other cases, the DRM is just used as an excuse to get the game for free.
There will obviously be some that will ALWAYS pirate, justifying it differently every time. And pretty much all piracy is illegal, if not immoral/amoral (depending on laws, of course). So the question is not "is piracy morally wrong/illegal" so much as it is: "how much does piracy hurt? And who?" I suspect that it's not quite the devestating death knell publishers insist on (just look at the 80s to see that piracy is not the only reason for bankruptcy. I mean, the market's going to stop supporting shovelware at some point.) and not quite the harmless pleasure many make it out to be.
There have been studies, but pretty much all of them have been funded by big publishers, with small sample sizes, cherry-picking and flawed variable control (very difficult to truly accomplish in such an inherently uncontrolled environment). So the question's still up in the air.
Personally? I have enough disposable income (I mean, I can afford to build a $1000 PC every couple of years. That's got to be above the norm) and Steam sales on games I'm into happen often enough that piracy just isn't an appealing option--too much risk and not enough reward.