Ok, so unions aren't designed to make companies not be evil. They're designed to increase comp. So since you have stated that Google pays lots of money, what's the point of a union.
The manifesto for the one union which attempted (is attempting?) to organize at GOOG is explicitly trying to reducing what they perceive to be evil, and aren't trying to increase comp.
Unions are also used to make working conditions safer, fairer and less exploitative. While money is definitely a key part of unions, it isn't the be all and end all.
