Do unions increase productivity? | Free exchange | Economist.com: "Some thoughts on markets where unions will produce higher productivity:
There are opportunities for deploying capital to replace low-skilled labour
The union wage is higher than the average prevailing wage for the workers' cognitive endowments and/or educational level
There are significant transaction costs to finding and retaining labour, such as the construction trades, where it is more efficient to call the union labour hall and tell them to send over 50 guys than hire them individually
The work easily lends itself to classification and regularisation
Productivity is easily measured"
In interesting article to read not long after Steve Jobs' speech against teacher's unions and my post on measuring worthwhile things.