If after playing through a chunk of the game you find it to be so terrible or boring that you can't even handle playing it through, I highly doubt that the rest of the game would have painted a different picture.
Let me give you an example: Glover. It's a pretty mediocre game, where you fight through some levels so that you can restore some wizard's magic and stop your turned-evil glove. I played through the first four worlds before getting so utterly bored that I used cheats to see the other worlds.
You never get any upgrades, there's never any interesting changes in game mechanics, and the bosses are always annoying. I eventually looked up the ending and discovered that it's even less rewarding than the playthrough itself.
So what, since I didn't sit through every single level and every single boss, I'm not entitled to think it's a mediocre game?
Or hell, Quest 64. It's repetitive, cutscene-less, story-less, and if you didn't talk to anyone besides the required starting text and bosses, you wouldn't even know wth was going on. The game's abysmally long and offers no help whatsoever. I came close to beating it before my memory card became corrupted and deleted the save. It's a bad game. Me not finishing that last 10% does not make it any less true.
Similarly, a reviewer should give it their all. Keep in mind that a reviewer is playing games because they have to, not because they want to. They might not like the genre they're being forced to play, the controls might be retarded, the story might be bollocks, and a whole bunch of other factors. You the gamer pick out games that you think you'll like based on what the game is about and what the gameplay is like.
If you're a JRPG fan, for example, you don't go and watch Yahtzee to see if he thinks your game is good. You know he hates all JRPGs. If you're a Witcher fan, just give up: Yahtzee hates your game. It's not a crime that he couldn't make it to the end of the game. If I hate a game, I stop playing it. If reviewers were forced to drag themselves through games, even if they couldn't beat a certain part due to difficulty, their ratings would lower even further.
It's like when I started watching Inuyasha. I made it a few seasons in, and at that point decided that I needed to finish it. I was too deep in to stop. I struggled through to the end, hating every character, every decision, every joke, everything. You know how I felt about it at the end? I hated it even more because the ending was just a final pile of shit to add to the "fuck you" pile.
So no, no a reviewer shouldn't be forced to play through utter shit if they think it is utter shit. I figure this topic has two reasons for existing: Either you're a Witcher fan who is upset that Yahtzee didn't praise your Witcher series, or you're a DNF fan upset that reviewers everywhere hate it.
Witcher doesn't hold your hand, yadda yadda yadda. I know. I'm not a big fan of games that are dark, gloomy, miserable little piles of brown. I do like a vast amount of complexity in a game, and I probably would give the series a try, but the apparently huge emphasis on boobs turned me off altogether. I don't get what is so utterly amazing about the game.
DNF is shit. Everyone knows it's shit. The only people screaming that it isn't shit either never knew Duke, love them some modern FPS bullshittery, or are so retardedly immature that they find the decade old jokes hilarious. Quit yer crying. The results are in: the general consensus is that it is shit, and no, playing through the entire game will not make it less shit. Unless the final cutscene is Duke being forced to mature and realizing that his ego is too inflated for his own good, I don't see how the ending of the game will possibly be so utterly different from the rest of the game that it'll be worth struggling through all the garbled bullshit that comes before it.
Actually, there should be a name for this. The Final Fantasy 13 Syndrome. We can call it FF13S, or Ffies. Saying "it gets better" after playing it for some absurd amount of time when the first massive chunk of it utterly sucks is not a good reason to keep playing. A good game hooks you in from the start with the best of what it's got. Making you struggle through for 40 hours to get to the "good" part is bad design. A reviewer can safely assume a game sucks if anyone who isn't a diehard fan won't be clawing people down to suffer through a shitty beginning just to get that little nugget at the end.