I don’t think it’s necessary for a game reviewer to finish a game before writing a review about it. If a food critic started eating a meal and it was obviously terrible after a few bites, would you still expect them to finish the entire meal? No, they can obviously give a proper review based on the exposure they had up until the point where they decided “this is how I feel about it.”
Let me know if you disagree in comments, but also let me know why. (Or if you agree!)
UPDATE: Lots of great comments. I think my favorite one is from zquar so far. Keep letting us know what you think! UPDATE 2: A formal response was published here.