well you can pick and choose were the invisible hand has been efficient if you want.
unions are not perfect (my dad belonged to one and he bitched about it all the time), but they do serve their purpose. and what are the "evils" of unions? they appear most "evil" in places where they become corrupt and bureaucratic. when they become an organization like the mafia, in a parasitic fashion you describe...but that is just one side of the coin.
historically unions have done a huge amount for workers. sure you wouldn't like to roll back all the changes unions have brought to workers.
conversely employers can be much more "evil" and get away with it. the market has a STRONG demand (i.e. guiding invisible hand) for "cheap, I don't care how or where it came from" products. Look at the Gap, Nike, etc. Of course those people are overseas, so who cares, right? Should we wait around for people to stop buying that stuff because of the "injustice", or should they unionize? Which would do more damage to their well being? Hard to say if you ask me. But is would seem that unionizing has historically worked during certain periods, only to became evil later during more affluent periods.
i'm making all this stuff up.