If you make N small, some shares contributed wind up earning nothing.. but that's totally unpredictable and affects all shares equally, so there's no tilt towards any miner timing or scale there. It's perfectly fair in those dimensions.
It's fair in average. But not enough. People who can't for some reason (malicious or not) contribute at the end of a block end up getting nothing, whereas the score-based system still gives them a little something. While, technically, the method you suggested is fair, it can be very frustrating for an isolated miner that got absolutely nothing for his work.