What is wrong with downloading a game to test out its full capabilities, then decide whether you want to buy it?
If the owner of the game allows it, there is nothing wrong. If the owner of the game does not want you to, it is wrong to disregard their intellectual and property rights and forcefully take what you want.
So why can't we download a game in order to try it out, then decide after that trial run that we like it enough for the game to be deserved of purchase?
Because it is illegal (in most places).
why should those who download merely to try before they buy be looked down upon, just because they wanted to test for quality before investing their hard earned money?
Because they broke the law for no other purpose than their own entertainment (and possibly greed).
I myself believe that reviewers get paid under the table to make good reviews on some games, because in those cases I myself can't agree with a 9 out of 10, when the game only puts out a 6 out of 10 in quality.
Welcome to life, where people are allowed to have opinions. Game reviews, like movie reviews, book reviews, and reviews of every other form of entertainment media, are wholly opinion pieces. My suggestion would be to do more research, in the amount of time (with the same amount or even less effort) that it would take to find, download, and play a pirated version of the game you could check forums, other review sites, and the good ol' word of mouth approach to find what most people think of the game, and who are definitely not speaking for profit.
Rather than being about piracy, this thread is about the debate of demo's to try a game before buying versus downloading to try a game before buying.
I added the emphasis since that is the very definition of software piracy, obtaining and using a retail version illegally.
One can dress the issue up however they like it, but in the end it boils down to the simple fact that one is breaking the law for their own entertainment (or possibly greed).