In a market where there is an oligopoly, there also tends to be an oligopsony for employment market. An oligopoly is like a monopoly, but instead of one large company selling goods, it's multiple large companies selling goods. An oligopsony is a group of large buyers in a market, it is the "oligo" prefix equivalent for a monopsony, which is one large buyer in a market.
By its very nature, a oligopsony has near dictatorial power over what it pays for a good, much like an oligopoly has when it sells goods. If one of the oligopsonistic companies wants to pay $5 a hour for labor, there is so little other choice in the market to sell labor to (which is what employees do), that employees almost have to accept it. The sheer overwhelming size of the oligopsonistic company gives it more power when bargaining.
The corporate model tends to produce monopolies and oligopolies, and thus oligopsonies and monopsonies in the labor market. For that reason, labor should tend to form equivalent monopolies and oligopolies, to sell their labor as one unit or multiple powerful units.
This is why corporations fight tooth and nail to oppose unions and bust unions big time.
There are downsides to unions, mainly striking and possibility that one's life can be upended during that time.
I think if we were to eliminate the corporations, and thus the monopsonies and oligopsonies for labor purchasing they create, the need for unions would end.
Not before then, however. IF we keep corporations around as a nation, then we must have strong unions to counter the mono/oligopsonies of large corporations.
Terms in this post that may be unfamiliar:
1. Oligopsony:
http://en.wikipedia.org/wiki/Oligopsony2. Monopsony:
http://en.wikipedia.org/wiki/Monopsony3. Oligopoly:
http://en.wikipedia.org/wiki/Oligopoly