Assuming a critic has gone through the great bulk of a movie/game, I think they can offer up a proper review of the item in question. Have you ever sat through a movie/book/game that you absolutely hated... only to have the thing become surprisingly good at the very end?
I can't think of a single one. If my hatred has risen to the point where I do not wish to continue, there's absolutely no hope that a sudden increase in quality would change my opinion (GTA IV is the closest and that started getting better after the first act). At best, you'll have a charitable thing to say about the ending combined with the cynical observation that it was too bad the rest of it wasn't anywhere near that good.
A long time ago, a movie called "Little Indian, Big City" came out and its review print accidentally left out the third reel. Roger Ebert dutifully watched the missing reel when it was supplied to him and he quoted Gene Siskel with "If the third reel had been the missing footage from Orson Welles' `The Magnificent Ambersons,' this movie still would have sucked."
And I also believe that a reviewer is under the obligation to detail his thought process, even if it leads to him acting in an unprofessional manner. If the first 20 minutes of a movie are a completely vile mess of misogyny, racism, homophobia, etc. (like a Halo deathmatch in film form), then a critic would be well in his right to walk out and dutifully report exactly why he couldn't sit through another minute more of it. It's his job to tell you what he thinks, why he thinks it, and let you make an informed decision whether you want to follow his advice or not. If he doesn't generally like horror movies, then he needs to point out in his negative review of a horror movie that he's typically not a fan. If he dislikes remakes on principal, he needs to say that in a review of Rob Zombie's Halloween. It's his job to put forth an accurate representation of his biases, whatever they may be.
His employers dictate what sort of form his reviews take on. If they want a comprehensive review of the pros and cons of any given title, then that's what they'll pay for. If they want something more akin to entertainment with many valid criticisms mixed in, then that's what they paid for.