> Isn't that a desirable outcome ? Shouldn't the able bodied person who is working obviously earn more ? Giving people incentive to work not only helps them rise up but also prevents their exploitation in case they try going to underground economy.

This happens anyway in our current system. Someone who's disabled and unable to work already earns less than someone who's working 50 hours a week in a minimum wage job. Negative income exacerbates that.