There are definitely times when this solution can be advantageous: When the expertise is too rare and too costly to develop, or when there simply isn't enough capacity inside the agency to keep up. Good examples include using private collection agencies for outstanding fees and taxes, or employing private after-hours child-abuse investigation units.
In most cases, though, outsourcing isn't being done to supplement the work of government employees. It's done to replace them. The assumption behind most pushes for outsourcing is that the private sector can do it better. Many people hold this as a universal truth, that private sector employees are simply better and more efficient than public sector employees. (And after corporate America's stellar performance in 2008, who could argue?)
But is this true? Are those of us in government defective in some way? Have all of the slow, inefficient, customer-hating people gravitated to one industry?