This makes sense for a couple of different reasons. As a retained employee who saw layoffs, your efforts increase to ensure that you are not ousted during a future round of layoffs. Further, the work of the employees that were not retained can typically be divvied among other employees. A group may not have a manager, but a Lead Engineer who now has responsibility for ensuring time sheets are submitted in a timely manner.
ad·junct - /ˈajəNGkt/
Noun: A thing added to something else as a supplementary rather than an essential part. (See source.)
There are two significant issues I think. First, schools are downplaying the importance of their teachers. I will give you a second to digest that. The role of teacher/instructor/professor is no longer the important position in a school. They have an abundance of other staff such as human resources and information technology personnel that are all benefited positions. However, these positions are not the money makers. Which employees should have the benefits? I would think that you want a content faculty that looks forward to coming to work rather than a bunch of people that feel they are getting taken advantage of.
The other issue is the quality of the instruction that the students receive. I am not suggesting that all adjuncts do not teach to the best of their ability. I know quite a few that are phenomenal instructors. However, I have had the displeasure of being a student of lousy instructors. I believe that in many cases they are distracted by their full time profession.
It would be nice if schools would revert to the old standard. Bring back the professional teachers. And I say this at the risk of reducing my chances of teaching as well as losing my wife's employment as an adjunct professor at a nearby college.
Update: Changed secondary to post-secondary.