The other educator who has responded to this question has done a thorough job of answering it as it pertains to K-12 grades. I would like to answer it from my perspective as an English instructor at a university, as this conversation certainly pertains to higher education.
Many American colleges and universities are undergoing the same staffing troubles right now-- the adjunct crisis. Rather than hiring full-time employees who have the opportunity to work under a stable contract, receive benefits, and eventually advance through the ranks of academia, most universities are choosing to hire adjuncts: teachers paid on a class-by-class contingent contract which ends at the conclusion of the school term. It's estimated that three-quarters of American professors are actually just contingent faculty. That means that students are being taught by instructors who have no job security and no freedom.
Unlike most jobs, the work doesn't stay at work when you're a teacher. When you are teaching five classes of introductory composition and rhetoric a semester with an average of 25 students per class, you are coming home with 125 papers (six to seven page long each) to grade about three times throughout the semester. This doesn't include the other materials that require review: homework assignments, tests, quizzes, in-class writing exercises, etc. The workload is exhausting and occupies most of our time outside of school, particularly if you are attempting to provide genuine, thorough feedback to each student. Most adjuncts I know are so eroded by this process that they end up simply assigning numerical grades without written feedback by the end of the semester; while this certainly is harmful to students who are learning and need to understand what mistakes they have made as budding writers, we simply cannot afford to do otherwise.
Many adjuncts are prevented from teaching too many classes at a single university; this forces them to look at other local schools to subsidize their teaching load. Many of my colleagues teach five classes per semester at three different schools: three classes at two different community colleges and two classes at a state university. Add on three hours of round-trip commuting time between three different cities to each work day, and you will see how quickly your personal time evaporates.
Other adjuncts I know work side jobs throughout the year to supplement their meagre income, often in retail or at restaurants. In the last three years of teaching at a university, I made $15,000 per year; in other words, I have been living below the poverty line despite the fact that I possess a Bachelor's degree in English Literature and a Master of Fine Arts in Writing. I supplement that income by working three other jobs--answering questions like this one for eNotes, editing online curriculum for a local school, and writing copy for a local business' marketing team. I consider myself lucky because at least I'm not doing hard labor; at the same time, these intellectual efforts are draining on the energy I try to preserve for my students.
Ultimately, this hurts students who are paying more than ever before for a college education. How can I--or any other adjunct in the same position--be present to my work when I'm worried about whether I'm going to be able to afford rent this month?
So, to conclude, yes, adjuncts should be paid more because:
1. We're highly educated, highly trained individuals making poverty-level wages.
2. We receive no benefits.
3. Our job security is non-existent; our teaching position could evaporate at any time.
4. The quality and amount of work we're doing eclipses the microscopic pay we receive.
5. It's better for students when their teachers are in a stable financial situation.