I'd like to think someone would fully complete a game before running their mouth in general. But what do I know.
I've never really agreed with this line of thought, that you can't really have an opinion on a game unless you've played X amount of it (I'm going to ignore the part about X being the whole damn game for a second).
I played through the Fez trial, but didn't feel compelled to buy it from that. Now, the trial only gave me ~20 minutes of play time sure, but the idea of a trial is to make me want to play it past that point. If I don't feel like purchasing the game after that, then it's due to one of two scenarios. Either the game itself is just not for me (which is pretty common, even amongst some of the best received games), or the game should have been for me, and the trial has done a bad job of showing me that.
What wouldn't make any sense for me, would be to decide that I wasn't really having a load of fun playing the trial, but I should now pay to unlock the full game so that I can verify for certain that I don't actually like it much. I have a rather large stack of already purchased games backlogged, along with a decent number of games that I would like to try just because they look like they will appeal to me (Fez was in this group prior), and a few games that no matter how old they get I seem to be completely incapable of not returning to (i.e. Daytona USA). Any additional time and money given to Fez would be at the direct expense of these other games.
Do you complete every game you ever play? If not, would you say that you don't know enough about the ones that you stopped playing to have a decent sense of why you stopped playing them?
For me, I'm playing games purely for entertainment. Every minute I spend playing a game whilst not being entertained is a minute that the game is doing something wrong. I'm not trying to earn my fun here. This does not mean every game has to be an arcade game by design however. As a recent example I played the trial for Gray Matter last night, and whilst the game's pacing was rather slow and over an hour later when the trial had ended I still can't really predict how the full game will turn out, the important thing was that at no point during that time did I feel like I didn't currently want to be playing it. That includes the first 20 minutes. When the trial was up, I wanted to be able to play the next hour, and so it's a game that I will purchase. Pretty simple.
If every time I didn't like something I continued playing it I would not be trying out a large number of games that currently have more potential for enjoyment. Doesn't sound like a worthwhile gamble to me. I have tried this a few times (e.g the first Uncharted), and funnily enough the first 20 minutes had usually told me everything I need to know.