Liberals were once skeptical of public-sector unionism. In the 1930s, New York Mayor Fiorello LaGuardia warned against it as an infringement on democratic freedoms that threatened the ability of government to represent the broad needs of the citizenry. And in a 1937 letter to the head of an organization of federal workers, FDR noted that "a strike of public employees manifests nothing less than an intent on their part to prevent or obstruct the operations of Government until their demands are satisfied. Such action, looking toward the paralysis of Government by those who have sworn to support it, is unthinkable and intolerable."
More here and here. More complete quote from FDR below (FDR to National Federation of Federal Employees, 1937):
Also, unionization typically leads to higher salaries and benefits for employees, which is generally laudable, but with an important caveat: the more expensive you make something, the less of it you can have. If cops and teachers cost $150K a year, you can't have as many of them--at least not absent massive tax increases that will cause businesses to expand outside the state, thereby harming immigrants and poor persons who rely mostly on the private sector for jobs.
Also, isn't it generally better to have more teachers and police officers than fewer of them? If so, the more benefits and money you give them, the fewer of them you can hire down the road, especially if you're spending hundreds of millions of dollars a year on retired/non-working officers and teachers (in the form of pensions). By switching government workers to 401(k) plans rather than pensions, the same money we're using to pay non-working government employees could be used to hire more teachers and police officers and to pay them higher starting salaries.
One last point: when government unions cause a significant portion of their members' compensation to be back-ended, i.e., in the form of pensions and lifetime medical benefits, you have two major problems: one, the politicians involved in negotiating the promises won't be around to suffer any consequences if they made unfair, overly generous, and unsustainable promises; and two, budget planning becomes very difficult because governments are not life insurers and cannot accurately or fully predict the costs of their employees' lifetime health care and pension benefits.
[This post was updated on June 8, 2012. More here, from Bruce Bartlett.]