Perhaps the most damaging change in higher education in the last few generations has been the wholesale shift in the composition of the teaching staff. Formerly, full-time, tenured faculty members with terminal degrees and long-term ties to the institution did most of the teaching. Such faculty members not only were free to grade honestly and teach with conviction but also had a deep understanding of the curriculum, their colleagues, and the institutional mission. Now undergraduate teaching relies primarily on graduate students and transient, part-time instructors on short-term contracts who teach at multiple institutions and whose performance is judged almost entirely by student-satisfaction surveys... Contingent faculty members, who are paid so little, routinely teach course loads that are impossible to sustain without cutting a lot of corners.
I'm of two minds (or maybe more) on this. One mind is in total agreement. As an undergrad, I attended a tiny SLAC where seminars might have had two or three students, and we were always taught by full-time professors, who were almost always available to students outside of class. As a grad student at a research university, I was a TA where, with no training whatsoever, I was grading papers and exams for a class of over 100 students. My first teaching job was an adjunct position at a SLAC. I was hired at the last minute, and had no prep time -- I pretty much had to prepare each lecture the night before class, since I was simultaneously taking classes and working as a TA. That was not me as the best teacher I could be. It was me as the best teacher I could be under extremely difficult circumstances.
The other mind knows that adjuncts, non-TTs, and VAPs are exploited as cheap, abundant labor, and Benton is right that as contingent workers, we can be worked to death and pressured to please students. As part-timers and temps, we have little connection to the institutions and departments we work for. The kind of personal interaction and sense of place and tradition that I benefited from as an undergrad isn't available to the students of adjuncts.
The other mind thinks adjunct teaching should be an important part of graduate education, since many philosophy grads do aspire to an academic career. But with so few permanent TT jobs available now, a substantial number of contingent faculty will never get TT jobs, so they are, in essence, being strung along in the vain hope that there's a permanent job out there for them. But the numbers are against them. (Granted, some adjuncts may not want TT jobs.) Eliminate most adjuncts and non-TT positions and there are fewer philosophy jobs to go around. But maybe that's better, both for students, and for the exploited workers.
Except that I gained a lot of valuable experience as a teacher while working as an adjunct, and that experience has made me a better teacher. Take away all that experience, and I'd be an inexperienced teacher if I was lucky enough to land a TT job.
How's this for a solution? Grad students who aspire to teach ought to be mentored in teaching, and instructed in teaching, just like education students are. Make teaching instruction part of the graduate curriculum. Give grad students real experience teaching classes in grad school. Then the argument that the exploitation of adjuncts can be justified because valuable teaching experience is gained evaporates.
The downside (or upside): more of us can figure out sooner that there aren't really enough jobs in philosophy to go around.