CFI,
point taken, however when hiring an aircraft to fly solo, it is fairly rare that you will then get charged by airswitch/flight time. Most of the time you still get charged VDO even though there isn’t an instructor beside you.
This means that while you are letting the engine warm up, completing your checks/run-ups and taxiing to/from the runway, waiting at the holding point due traffic/ATC, you are getting charged lets say $150/hr. During this time, there are no maintenance costs being incurred by the company hiring out the aircraft and the fuel burn at idle is close to insignificant. I’ve seen many people try and overcome this by taxiing fast, breaking hard, rushing checks etc., just so they can shut down before the VDO clicks over into it’s next digit. Sure it is poor airmanship, but the temptation is there for some people.