Most of the drilling I've done in the past is concentrated on sweeps, passes or overall body movement. I've never seen a mount to side mount and back drilling done before but this must be a good idea since these transitions are often performed very quickly to counter something.
Andre Galvao said that in his class they drill 80% of the time and spar the remaining 20%. I guess the warmup is drillling as well. In a normal class of 1,5 hours the sparring would only be three 6-minute rounds. I know this might sound boring but I guess this way people would get the necessary repetitions done..