Bad ideas are a dime a dozen. It's when political systems are based around bad ideas that the problem begins. You can argue with a bad idea, but you can't argue with an institution. All you can do is argue around the institution. Wisconsin gave us a showdown between a state facing a cash crunch and a public sector union. It was not a battle of ideas-- but of power.
The modern American union gives the left a cut of every unionized business. It forces the employees and indirectly the owners, to put money into the party coffers, vote for their causes and donate to their candidates. The union is sacrosanct to the left because it shifts power from the owners to them. If unions actually shifted power over to the workers, the left wouldn't bother with them. Unions that did that would be their worst nightmare. The Democratic party values unions as organizing machines, not out of any romantic notion of workers' rights.