More lean muscle can be correlated to organ reserve, less injury prone and higher metabiolic rate. If yuo are injured your chances of full recovery are increased.
When do HIV patients die? WHen they have been Ill so often that they have wasted away losing most of their muscle mass, correct? THey hit a tipping point and muscle mass and organ reserve are not sufficeint to fight the illness and maintain body function and they die(this could be said of most terminal diseases).
Let's say you get TB followed by the flu. Lean tissue is a buffer against it killing you. Lack of lean tissue is less buffer increasing the likelyhood of the double wammy overwhelming your body and killing you.
THis is why I don't mind my BMI pointing towards me being (overweight) despite the fact that I am fairly lean.
Integrity is what we do when nobody's watching.