Part of the frustration with adjuncting (and I suppose with many faculty jobs these days) is the need to learn something new in order to teach it. It's not that I am in principle opposed to lifelong learning (to the contrary). But there is something false in a teacher keeping one or two steps ahead of the students. There's something inappropriate about being hired to teach a course I am not expert in, when there are so many experts in the subject who could teach it.
My Wednesday night class is of this sort. I've never taught it before (a fact which was well-known to the committee that hired me). Sure, I have a knowledge base to draw from, and astute comments to add to the discussion, metaphors and illumination to contribute to the readings. But I tire of the effort to produce a lecture from material that is unfamiliar to me. At least, because my interests and my heart lead me elsewhere just now.
But why do they hire a non-expert? Because the priorities are skewed. Class sizes should be smaller. More courses should be offered. More faculty should be hired. Those best able to teach a subject should be used whenever possible. The sad thing is that my leaving will have no effect on this state of affairs, so long as there is someone else willing to step in and fill my shoes.