If we can rely on a shortage of desperate workers, we don't need to send mafia goons after them.

The ethical, nonviolent way to reject bad pay or working conditions is to quit, accepting that the employer might find someone else. That's not what unions do.

The historical behavior of unions is commensurate with the historical behavior of employers and strike-breakers.

Your adversary is not going to be ethical and nonviolent.

Unions resolve the prisoner's dilemma in favor of the prisoners. The game is set up like this:

  In each trial, 3 players distribute $300.
  A and B vote on whether E gets $100 or $150.
  E can cast a tie-breaking vote.
  E decides how to distribute the remainder to A and B.

  | A gets | B gets | E gets |
  +--------+--------+--------+-----
  |   $100 |   $100 |   $100 | A $100, B $100
  |   $150 |   $  0 |   $150 | A $150, B $100
  |   $  0 |   $150 |   $150 | A $100, B $150
  |   $ 75 |   $ 75 |   $150 | A $150, B $150

Obviously, if A and B cooperate, and voluntarily form a cartel with the power to enforce cooperation, they will experience a better outcome. Because in the long run, in repeated trials, the employer will be pocketing an extra $50 in a huge number of trials, while pitting the employees against each other.