The CPH metro also has short trains - evacuation of a three-car train is substantially less difficult than the safe evacuation of, say, a Victoria Line train - 133m long, with a crush load of 1,200 people and no side walkways. You're right that the benefit here isn't so much in cost savings (most trains still have at least one staff member on board) as it is in being able to run short trains at low headways.