They produce minimal value to society while necessitating the upper range of an average person's resource consumption in order to keep them alive and stable.
They are a burden whose offspring will likewise be disadvantaged in society.
In the metaphor of cells, mentally ill persons are akin to cancer cells - where in the cellular microcosm they are eliminated by the body's immune system, we as a society tolerate and even at times celebrate the mentally ill.
To execute and harvest the organs of a MIP would be a net benefit to society and the individual, who is freed from suffering.