Except lean muscle is a metabolically healthy body tissues and every pound of lean muscle you have would help forging a stronger, physically tougher body that allows you to not only look young, attractive but also gives you the raw strength to complete more physically demanding activities, and for some allows them to be more competitive in sports for instance. And having a stronger body generally gives you more confidence and helps boosting your self esteem which can in turn affects your mood and quality of life. And girls tend to like fitter, swole guys over scrawny guys.  So think again when you say it's "ineficient" for guys to be working hard in the gym and eating right to build strength and muscle. You choose what you want to do, but don't go around putting a negative connotation about it. You like being medically underweight, go ahead.