The New York Times ran an article a few days ago lamenting  the decline of tenure track positions in American universities. While frustratingly indicative of a fall in teacher quality and a high turnover of part time and non-tenure-track university professors, advocates forget that a high number of tenured professors probably will not improve teacher quality.
Teacher’s unions fight for tenure because it supplies the ultimate job protection; after teaching for a given number of years a teacher simply cannot be fired. Visiting professors might only stay for the year but can carry the hope and the incentive to do well, in case they might be offered a tenure-track position. Tenure-track professors will work hard too because they are faced with the incentive of increasing their rank and achieving job security. Tenured professors, on the other hand, have lost all incentive to perform at a high level. University professors are also different from teachers in lower education; many, especially at larger institutions, enter the profession not because of a drive to teach, but often because they had completed advanced study in an area and needed a job. Once offered tenure, these professors might stop teaching all together in favour of their research or publications.
Adam Smith said that teachers must have proper incentives of pay and job security in order to properly instruct students; anyone whose pay is not linked to their work will necessarily under-perform. The ability for improvement and the threat of pay cuts improve any profession, and teaching is no different. Professors must always have opportunities to advance, not a position which completely removes the incentives to do their job well.