A big advantage of the negative income tax is that it never penalizes you for earning more money. Even with the supplement, you're still always better off the more you earn by working.
Under the example scenario above,
An unemployed person makes $20,000 a year
If he gets a $20,000 job he makes a total of $30,000 a year
If he were to get a $40,000 job, he'd be making $40,000 a year