in the pursuit of absolute airflow, don't lose sight of the purpose of the screen - it's there to spread the average airflow over the full area of the MAF tube so that the sensor portion in the centre will always be measuring an airflow value which assumes the whole tube is flowing evenly, this may be more significant at low flowrates, where otherwise the tube may not be flowing uniformly. Picture if you will the situation where the air may be flowing in an area of the tube not occupied by the hot film sensor in the centre, there is clearly airflow in the tube but the sensor would not "see" it, hence potentially producing fueling anomalies due to the non-uniform flow across the tube area.