I'm currently part of a consulting firm. It essentially manages and provides labor to paying customers. It provides benefits to the maximum extent allowed by law. That is, we are often frustrated by the law when we want to provide better benefits. One example is retirement savings. We would provide more, but the law prevents us.
If an employee is doing good work, the customer paying for it will let us know. If an employee isn't, the customer will let us know. Most of the time, we'll find a better fit for that employee. Rarely, we'll let them go. When an employee wants to move on, they can find another position within what we offer or they can find new business and bring that in. They'll get a share of that.
To be honest, this sounds like a (more) authoritarian version of a union, where it's led by a dictator/owner rather than an admittedly flawed democracy, and capable of the same flaws unions are vulnerable to.
For workers, what makes this ambivalent dictatorship arrangement categorically better than a union hall?
I think any honest examination of the question is going to conclude that employers and their employees often have opposing interests and the lack of collective bargaining makes it much easier for employers to have their way in a relationship that is wildly imbalanced in the first place (I mean, really, compare the stakes of one worker's job for the employee and employer). We are in a profession where we are far more insulated from the ill effects of this than others; I think a lot of people posting here forget that.