I don't think that many people believe that unions were always unnecessary. They were vital in a time where employers could just do what they wanted. Nowadays, though, unions are built on greed and laziness.
That's certainly the perception, but I still have to wonder how true it is vs how much is propaganda. Still, even if it is true that unions weren't necessary for a period of time, they most certainly have become necessary once again:
And big companies aren't? They aren't built on greed and squeezing every last bit of productivity out of someone for the absolute minimum they can pay?
Seriously, the only reason people have that perception is because big companies have the money to run smear campaigns against labor.
http://www.forbes.com/sites/scottwinship/2014/10/20/has-ineq...
How else do you interpret "the wedge" except as the failure of labor to advocate strongly enough for its interests?