It takes a lot of force to drive a blade through mail. The blades aren't sharp enough to cut the material so what would break it is blunt force. When you consider a bullet from a gun doesn't have much more force than a human punch but it's focused much smaller than a fist and can cut through flesh with relative ease, getting stabbed in the gut hard enough might end up pushing the chainmail into your flesh. It turns a knife into a narrow, but blunt object. I think it would definitely up your torso's chances of not getting cut.