I think given that game developers these days are releasing more and more buggy, untested and unfinished games that often outright lie about minimum PC requirements it's not surprising that piracy is becoming more and more acceptable to test whether a game is even playable let alone good.
I certainly wouldn't buy a game unless I'd played it first.
That's true enough, but if you take that point of view you might end up like the pirates in that other post about "copy-protection biting them in the butt" - can't be bothered to go back and get the actual post...

I think the point is that if there's a type of copy-protection on the game (obviously Sins doesn't, but this is an example) that checks
throughout the gameplay itself whether or not the game is pirated, and, if pirated, it dumps the game, then pirates are going to be thinking "gosh, this game is buggy - I'm not going to buy it," when in actual fact the reason it keeps crashing is BECAUSE they are pirates, and wouldn't do so in a legitimate copy.
In other words, something may come up on a pirated copy that you, the gamer, might see as a bug with the game anyway, but may be an artifact of the copy being pirated...
I don't break the law to see what a game is like. I read reviews. Of both users AND game sites - it provides a good view. Obviously, you'd ignore user reviews like "I hate this game coz it's boring" and look for the reasonably constructed ones.
And if there's a demo, I'd download it