I think this summarizes why public unions are bad idea (a reality being lived out in Wisconsin and Ohio right now). From the Journal:
Public unions have a monopoly position that gives them undue bargaining power. Their campaign cash—collected via mandatory dues—also helps to elect the politicians who are then supposed to represent taxpayers in negotiations with those same unions. The unions sit, in effect, on both sides of the bargaining table. This is why such famous political friends of the working man as Franklin Roosevelt and Fiorello La Guardia opposed collective bargaining for government workers, even as they championed private unions.