Speaking as a player I rarely finish entire games because they are too long and can't hold my attention. I have started and stopped "Oblivion" maybe 8-9 times and have never seen the end. I have played more than 50 hours of "Fallout 3" without seeing the end. I have never played through "Diablo II" regardless of how many characters I have created in it. And I like those games.
I have rented or purchased games that have been so bad that I could not bring myself to finish playing the first level/chapter/mission. It is not necessary to play a game through to know it is bad or good, and I can give you examples: "Superman" for the Nintendo 64, "Peggle" by Popcap, and "The Simpsons" for the original Nintendo Game system.
If a reviewer dislikes a particular game style, be it FPS, MMORPG, RTS, or my personal bane TBS, then they can tell whether a particular game is going to turn them around on the subject without seeing the whole narrative.
I personally could not have finished "Doom" or "Quake", I played them on the easiest mode, took the games at a run, and just could not finish them. They stopped being fun, and while I could say that I got numerous hours of enjoyment from playing them, I didn't need to see the end to have gotten that positive feedback.
In my eyes, you can tell something is a turd with one bite, you need not finish the meal at that instance to give the review of "That was a piece of shit".