No one is worthless. Power exchange dynamics cannot ignore mental health, and we must temper fantasy with reality. subs are not worthless; I would argue that if a sub was indeed worthless, they would be of no value to a Dom. In fact, a worthless/valueless individual would be a liability to anyone they enter into a relationship with.
Know your worth, know your value, and guard your mental health. It's just as precious as your physical health.