propane tank purging

I just took my cylinders in to refilled and the attendant started to purge them. When I asked why he told me you get more propane in the cylinder. That made no sense since the propane is liquid and any air would be forced out of the vent hole. I insisted he stop. I do know that scuba tanks should never be run to zero pressure because atmospheric air can get in and cause corrosion issues and I assume that holds true for propane cylinders as well.
RichH