The three main reasons they gave were: 1) a functioning free market with rational agents voluntarily exchanging goods and services requires non-desperate individuals who can realistically say "no" to proposed economic exchanges; 2) a guaranteed minimum income would make people more autonomous market participants, as they would feel more free to be entrepreneurs, and to break from more cliquish kinds of safety nets like churches and ethnic groups; and 3) pragmatically, direct cash transfers are the least distorting kind of anti-poverty program, so a minimum income will greatly reduce political pressure for more distorting measures, like restrictions on when you can fire employees.
In Milton Friedmans capitalism and freedom, he discuss (among other things) that proposal - he admits not liking it too much, but it is better than the current system of welfare and it is more realistic to create that than to kill the welfare system.
In addition you save the expenditure associated with paying for the enforcement, social workers, etc.
