From Merit, Tenure, and Bureaucratic Behavior: Evidence From a Conjoint Experiment in the Dominican Republic, Schuster and Oliveros concluded that merit based recruitment (examination) enhances the quality of bureaucracy, in this case, enhancing work motivation, lowering corruption, and also enhances the democracy (electoral competition). Job stability, however, only enhances the quality of democracy. Because merit based selection creates a more politically neutral public service, which will be less likely to help parties with electoral efforts.
I thought this article was really though provoking because of the paradox it found to be true. While the authors found that job stability only increased the quality of democracy, they also had data to support the idea that tenured bureaucrats tend to be less effective.