Pure speculation: The hole is for, as you said, pressure differences. The head uses air pressure to float above the platter. Changes in pressure could bring the

Pure speculation: The hole is for, as you said, pressure differences. The head uses air pressure to float above the platter. Changes in pressure could bring the head closer/farther away from the platter which could have unwanted consequences. Pressure fluctuations are due in part to changes in temperature. Since the temperature is regulated by the oil, and that temperature will change more slowly than with air, plugging the hole is relatively safe.