There's a recent study of professors at Northwestern University that purports to show that students of adjuncts there learn better than those of tenured faculty in introductory-level classes. Jordan Weissmann at The Atlantic has extrapolated from this inconclusive study of one non-representative institution that adjuncts definitively make better teachers because they devote all their time to teaching, rather than research. The implication he highlights, of course, is that adjunctification is all hunky-dory because it means better classroom instruction. Others like Pan Kisses Kafka and William Pannapacker have already given this "pipsqueak" his due, but I can't let this go without offering my much less respected opinion.
As someone who has taught as a TA and both on and off the tenure-track, I've seen that reality is a lot different than this. In the first place, Weissmann bases his thoughts on the totally erroneous assumption that adjuncts and other contingent faculty aren't focused on research. When I was a visiting assistant professor I spent two years busting my hump to get articles out in order to get a tenure-track job. Perhaps more time consuming, I had to put together over thirty job applications a year, which was like its own part time job in itself. During my second semester, I made the conscious decision to put my teaching on the back burner because if I didn't, I would not have been able to write the conference papers and publications necessary to get a better job. My teaching suffered, but I had built up enough goodwill from my first semester that I knew I wouldn't lose my job, and the following summer I got an article accepted into a top journal that helped propel me on the job market to a tenure-track position. A second article followed in the early fall, and I was able to put more emphasis on my teaching. I felt horrible for short-changing my students that one semester, but it had to be done, and was a conscious choice that many adjuncts and "visitors" must make.
Despite those pressures, I've known many contingent faculty who were fantastic teachers. Some of them were high school teachers who had retired or who were moonlighting, and so had a tremendous amount of classroom experience. With the contraction of the tenure track job market, there were also a lot of folks who had the bona fides to be on the tenure track, but who had been done wrong by the system. (I fancied myself one of these types.) I published more in my two years as a visitor than many fullprofs in my department had done through their entire careers (giving lie to Weissmann's thesis, which only holds for research universities.) I have multiple friends who have published well-received books with good presses while working as contingent faculty, and who also happen to be fine teachers. Their abilities have nothing to do with having more time for teaching, and everything to do with the fact that they are talented people who ought to have better jobs.
Despite the number of great teachers off the tenure track, many of whom are simply better at their jobs than those with tenure, there are plenty who struggle. Much of this has to do with the privations and stress of contingent work. Many adjuncts must cobble together classes at multiple institutions, leaving them stretched thin and worn out from their commutes. Others have to work side jobs to make ends meet, like a friend of mine who worked for a wine wholesaler while adjuncting four classes. Like me, he had needed to do emergency dental work, and adjuncts and visitors alike at my institution did not have dental insurance. The expense meant that he had to take additional work, and I had a great deal of added stress over finding a dentist I could afford who could help me.
By and large, adjuncts get little to no institutional support. Many don't even have a desk or office space, or must use spaces that are humiliatingly shitty. There is almost no mentorship for young contingent teachers, and often no oversight, either. Along with the great contingent teachers I've known, I've seen others who were horribly irresponsible. One I knew had carried on sexual relations with a former student (while she was still enrolled in the university) and once left for a week to attend a wedding, and had one of his students show films. Another was totally unqualified to teach world history, and so merely read aloud from the textbook. His evaluations were horrible (for the right reasons, this time) but he kept his job because the institution needed a warm body in the classroom. My friends who busted their butts got so demoralized, since we were all paid the same amount of money, and all given the same amount of recognition: zilch. It quickly became obvious that the only thing keeping me responsible in the classroom was my own conscience. That is hardly a recipe for quality education, and certainly breeds feelings of cynicism and even despair among contingent faculty.
Adjunct labor is a system that is rotten to the core, and it's only getting worse. Like all other wealthy industries, higher education wants workers who are powerless, replaceable, and above all, cheap. That is the only real reason for the rise of contingent labor in the academy, and to imply that this is possibly a good thing and good for learning is sophistry, cant, and utter bullshit of the worst variety.