More from the folks at the Volokh Conspiracy on administrative bloat in universities. The comments are interesting too, particularly the ones from "PatHMV".
I think an alternative view of the gradual increase in the number of lecturers is that it may make sense on several levels for departments to have a set of lecturers who specialize in teaching and to assign those individuals to teach the introductory classes where the material is relatively standardized and (typically) far from the research frontier. The university saves money and the students often get better teaching.
There are, to be sure, complications. Some (but not all) of these lecturers might want to be researchers, and so may become unhappy and bitter if they end up with a career as a lecturer. Careful hiring can avoid this outcome as can added doses of self-knowledge and realism among such lecturers.
In the other direction, tenure track faculty have to avoid treating lecturers as second-class citizens rather than as people doing a different job than we do. Lecturers should be judged on their performance at their job, not on the difference in content between their job and that of a tenure track faculty member. Put somewhat differently, the tenure track faculty should, in both a normative sense and in an efficiency sense, treat lecturers just like they would other professionals, such as the department IT specialist or the department senior administrator.
Lecturer unions, which are more common (by a long way) in the US than unions of tenure-track faculty, are also a problem. Their existence surely has slowed the trend toward the use of non-tenure track faculty relative to what it otherwise would have been. As unions often do, they reduce flexiblity and reduce variance in pay. This makes it hard to hire lecturers in economics, where potential lecturers have good outside opportunities and so should be paid more, and essentially impossible to create positions intermediate between lecturer and tenure track faculty, as one might on occasion want to do.
Who was my favorite student this term?
10 months ago