Actually, with an open head, you wouldn't get reversion, you'd get diversion, the sound energy from the exhaust would go wasted as it's released in a chaotic manner. This obviously does nothing to help scavenging, so what do you get? You get no thermal barrier in place for the valves since there's no direction to the heat and sound to create a rolling "wall" of air that helps scavenging, and instead you can allow ambient temperature air [which is much lower than the combustion temperature] to make contact with the valve face, which can cause all sorts of nastiness such as warping, and that air can also carry contamination into the engine as was said earlier.