To clarify. I never said they never contributed to productivity. At one time they did, but these days they have largely succumbed to the shirky principle:
“Institutions will try to preserve the problem to which they are the solution.” — Clay Shirky
The same can be said of the organizations that have typically opposed the unions too. The world has too many zombie organizations that have outlived their usefulness and are now a drag. The effort being spent on unions would be better off spent on increasing support for a guaranteed minimum wage. We've now got universal healthcare (or at least some flavor of it), but there are a lot more ways we can improve equality of opportunity and fairness and I don't see how the modern unions contribute in any way to the bigger picture (like the 8 hour workday)
Productivity is what allows more and more of us to pursue activities that make us happy for more hours of the day.
http://kk.org/thetechnium/2010/04/the-shirky-prin/