Of course unions have pathologies. Every big human institution does. And anyone who thinks they're on the wrong side of an issue should fight it out with them. But unions are also the only large-scale movement left in America that persistently acts as a countervailing power against corporate power. They're the only large-scale movement left that persistently acts in the economic interests of the middle class.
So sure: go ahead and fight the teachers unions on charter schools. Go ahead and insist that public sector unions in Wisconsin need to take pay and benefit cuts if that's what you believe. Go ahead and rail against Davis-Bacon. It's a free country.
But the decline of unions over the past few decades has left corporations and the rich with essentially no powerful opposition. No matter what doubts you might have about unions and their role in the economy, never forget that destroying them destroys the only real organized check on the power of the business community in America. If the last 30 years haven't made that clear, I don't know what will.