Well the point is that with the QBZ your check cannot ever get low enough to rest on the stock. You therefore repeatedly introduce an error in aim that gets magnified the further out you go. The rifle's zero depends on consistent alignment of 3 points: your eye, the back sight, and the front sight. If the position of your eye can wander from shot to shot as it's more likely to do with no cheek weld, your accuracy suffers. Cheek weld is one of the most basic fundamentals of shooting, along with breathing, trigger control, natural point of aim, etc. This is less important in a combat situation where you have to shoot and scoot, suppression fire, urban combat situations, and so on. But if you want to be accurate with any kind of long range shot, get a good cheek weld or go home.
