Talked about this to an engineer, (B747-400). Engineer said that the unit, for it is not so simple as a bulb/globe, is itself very expensive and they will fail so reducing their 'on' time does make economic sense but also to access the unit requires specialist ground equipment to reach the tail plane/stabiliser so labour costs escalate. I've worked for a carrier where they were switched off at 10,000' on the way up and back on at 10,000 on the way down, also worked for another major who left them on all the time.