Overcoming capitalism? Not really. They’re just one way of remedying the power imbalance between the management of the corporation, which by its nature acts collectively, and individual workers, who have considerably less power unless they can also act collectively. Running society fairly? Fair is a loaded word. “In the interests of the majority” is not something I’m especially comfortable with either; given that it only takes a simple majority vote to unionize a workplace, I suspect that if the majority of workers perceived joining a union to be in their best interest, there would be a lot more union members. (The problem here is the complex interplay between what a person’s short-term interests and long term interests are, and on what that person perceives his or her short- or long-term interests to be.) On the other hand, if you limit “running society fairly in the interests of the majority” to “decreasing the gap in salary/income between workers and management,” I’d buy that. When the management of the corporation has the ability to bargain collectively and the workers must bargain individually, the profits from the work go disproportionately to management. When both sides can bargain collectively, the proportions are more balanced.