Most diesels like to run at about 80% load. If I had time and just on a leisurely transit I would rather one one engine to keep some load on than two at a very low load each. In the ‘80s my two screw US Navy Destroyer frequently went in “trail shaft” mode to conserve fuel if we were not in a rush. Eco speed (most economical speed) has been the way of the commercial world for many years now. So yes, it’s horses for courses, but if all the right aspects of the operation are there it is a perfectly reasonable operating mode.