The knock against adjuncts has always been that they are with the school but not of it—they teach their classes and meet their office hours, but they lack a broader commitment to the institution. Having too many adjuncts is bad for the institution, because there aren’t enough full-time staff to do the other important work of university. I think this argument is oversold, but let’s go with it for now.
At the risk of over-simplifying, full-time faculty members see themselves in one of two ways. Either they see themselves primarily as mathematicians, sociologists or historians, who happen to work for a specific university (Type 1). Or they see themselves primarily as faculty members of the university, who happen to teach in the mathematics, sociology or history department (Type 2).
Colleagues holding the first view see their job as consisting of teaching their courses well, doing their research well, and serving on enough committees to “satisfy” the requirement for service. For many such faculty, one’s professional responsibilities can be summarized as a checklist. Service is seen as an obligation, rather than something bigger. I think this view is similar to that of adjunct faculty members described above, albeit with a somewhat broader but still limited vision.
The second view holds that a major component of one’s job is responsibility to the institution, to do what one can, to do what is necessary to help the institution flourish. This responsibility is not a ball & chain which keeps one from one’s real work—it is an essential part of the real work. That doesn’t mean such faculty are not interested in teaching and research. It just means they have a broader vision of the job. It is a vision which comes with tenure and experience (I’m not criticizing junior faculty who don’t have this broader vision).
A university desperately needs faculty who think that their job is greater than the sum of teaching, research and service, narrowly defined. Or putting it differently, a faculty member can satisfy the checklist (teaching, research, and adequate service) and still not satisfy what the university really needs. Type 1 faculty are like the caricature of adjuncts I started this post with. A university can afford to have some, but too many will prevent the university from flourishing.
For the record, I think that most adjuncts do a fine job given the constraints they work under.