All the talk of unreliable reviewers and lack of demos put me in mind of an article I read a while back, which I can't find now, but this snippet from
this Wikipedia page pretty much covers it:
The paper by Akerlof describes how the interaction between quality heterogeneity and asymmetric information can lead to the disappearance of a market where guarantees are indefinite. In this model, as quality is undistinguishable beforehand by the buyer (due to the asymmetry of information), incentives exist for the seller to pass off low-quality goods as higher-quality ones. The buyer, however, takes this incentive into consideration, and takes the quality of the goods to be uncertain. Only the average quality of the goods will be considered, which in turn will have the side effect that goods that are above average in terms of quality will be driven out of the market. This mechanism is repeated until a no-trade equilibrium is reached.
As a consequence of the mechanism described in this paper, markets may fail to exist altogether in certain situations involving quality uncertainty. Examples given in Akerlof's paper include the market for used cars, the death of formal credit markets in developing countries, and the difficulties that the elderly encounter in buying health insurance. However, not all players in a given market will follow the same rules or have the same aptitude of assessing quality. So there will always be a distinct advantage for some vendors to offer low-quality goods to the less-informed segment of a market that, on the whole, appears to be of reasonable quality and have reasonable guarantees of certainty. This is part of the basis for the idiom, buyer beware.
This is likely the basis for the idiom that an informed consumer is a better consumer. An example of this might be the subjective quality of fine food and wine. Individual consumers know best what they prefer to eat, and quality is almost always assessed in fine establishments by smell and taste before they pay. That is, if a customer in a fine establishment orders a lobster and the meat is not fresh, he can send the lobster back to the kitchen and refuse to pay for it. However, a definition of 'highest quality' for food eludes providers. Thus, a large variety of better quality and higher priced restaurants are supported.
This would also seem to be the case for some of the more egregious examples already mentioned (many from EA) - the potential buyer is aware that they can't rely on the sales information (biased/paid-off reviews, poorly representative or even no demo, etc) and so treats the game as being of "unknown" quality. In line with the article above, this drives overall quality within the market down.
In this instance, pirating the game could be seen as an effort to get clearer, more transparent information about the potential purchase. The example of taking a car for a test drive was already given (although in this instance it's more like sneaking into the sales garage at night and waving a magic wand over your chosen vehicle, instantly duplicating a copy for you to drive around).
The danger is then, as others have pointed out, that you might decide (a) yes, actually, this car isn't a lemon, it's actually a decent car and (b) hell, why go and buy it now, I've got a magically-copied car of awesomeness!
Piracy allows trial of games that would otherwise be of uncertain quality. This makes the market work more efficiently. Piracy also allows good products to drop in sales along with bad products, however, because you're not going to buy a game you know is bad, and you probably won't buy a game you know is good but have already pirated. Probably. Individual morality will dictate, obviously.
So, the $64m question is: what's better for the market - asymmetrical information that drags down the overall product quality, or symmetrical information that drags down the total sales of any quality?