A turbo fan engine cannot use all of its available thrust in cruise any longer that it normally does at today's technologies, otherwise it would risk a meltdown of the internal hot engine parts and or operate inefficiently because the fan blades would be above supersonic speed. It however needs its full thrust for take off.
The Extra disused thrust however represents extra engine weight relative to the total weight of the engine i.e weight whose thrust does not contribute to the engine at cruise. That weight is weight that the aircraft has to carry around for the thousands of hours of flying the aircraft throughout and through its entire lifespan, representing a loss in total efficiency.
Given the fans in a modern bypass turbofan engine produces 54% of the thrust Could the aircraft be a lot more efficient if an extra fan was added to the side of the engine though with no engine, to be or powered by the disused torque of the engine at cruise. I know there are problems with adding gears or a prop shaft to spin the extra fan for some engines, particularly large engines but some engine designs can accommodate gearing. The extra fan would then provide thrust that mitigates the relative dis used weight