To compare numbers, one adjunct professor makes $2,987 for each of two classes she teaches per semester (the nationwide average pay). Let's say she is able to teach two classes each during fall, spring and summer semesters (which may not always be the case). Then her annual income would be $17,922 (less than most PhD students in the sciences receive for their stipends, even if they TA a course each semester). Compare this to a distinguished professor that makes $144,000 a year. Often, they only have to teach a single class (or none, for having a distinguished level) per year, and can spend the rest of their time conducting research, writing papers and getting grant funding, and becoming well-known authorities in their subject areas in the academic community.
I think that fixing this wage discrepancy, and everything else that goes along with it (considering adjuncts full-time if they are working more than 40 hours a week on teaching, providing benefits and including them in faculty meetings, etc.) should be made a priority in academia, especially because these people are still helping to educate our next generation of young minds, and are often better teachers than tenured faculty.