your argument is starting to sound like that scene in "Life of Brian" (and i'm paraphrasing because i don't have it memorized)

"Ok, apart from roads, schools, public safety, the aqueduct and sanitation, what have the Romans ever done for us?"

your argument also does not account for the fact that management typically intimidates workers into staying out of the unions, even when union representation is strong. by cutting out the dues of union membership, the intimidation of workers to accept more and more work for less and less pay will be more widespread.

look at the examples of caterpillar, hostess, tyson chicken and others where management took a hard line against the union, won the concessions, screwed over the workers anyway, then gave themselves big bonuses for doing so. and these are all examples where the unions were willing to limit themselves and their power for the long-term health of the companies they worked for and got screwed anyway. by limiting the power of unions, how are workers supposed to cope when management eventually screws them over? the answer is, they can't.