That is not true. Underemployment does not benefit anyone: it does not provide the employee with a living wage and results in an unproductive workforce. You can't change the fact that there is a minimum amount of money necessary for a reasonable existence, and going below it is just a means for temporary exploitation of workers, in some (although possibly farfetched) fashion similar to slavery.

Subsistence work with no possibility of improvement does not provide society with a net benefit, but only creates a class of permanent poverty marred by undernutrition and an inability escape the tactical issues of living paycheck to paycheck, and it's an especially important issue in a country such as the US where health benefits are few and time off is minimal.

If minimum wage is taken away, market force WILL drive down the wages, but there will be no social benefit, as it will only result in denying opportunities to a very large sector of society.