John Weidner (Random Jottings) expresses something I had in mind:
Suppose thousands, nay tens of thousands of Federal employees were replaced whenever the White House changed hands. The continuity of well-established procedures would be broken. But, so would the continuity of entrenched lethargy and indifference. The new people would lack much of the knowlege needed for their jobs, but they would also be willing to try new ideas, and would know how things are done in the private sector. Some of the new people would be incompetent or venal, but . . . well, you know where that one’s going.
Weidner also touches on the former practice of selling commissions in the British army. Sir Iain Moncreiffe (1919-85) made the interesting point that this meant most officers were able to resign in protest without endangering their livelihood. (Indeed in many regiments an officer could not live on his salary.)
See also this memoir by David Hardy. (local copy, just in case)