Reduced Takeoff thrust or power is used when there are no obstacles that may require the plane to use the whole amount of power available for the takeoff conditions.. .When u lose 1 eng, most planes will uptrim the operative engine allowing extra power output...also, the standard call out "max power operative engine is used".. .Keep in mind all airplanes are certified to safely climb thru all takeoff segments with one critical eng inop at max takeoff weight. If Density altitude is high, weight is close to MTOW then reduced TO thrust will not be used...for all other conditions, reduced thrust is an acceptable way of saving wear and tear of the engines.