The economic logic of replacing tenure-line faculty with contingent faculty isn't hard to grasp: you get roughly the same labor (or at least the same teaching labor) for less money, and you can hire and fire at will. Duh.
But nothing brings that home like calculating one's own "replacement costs," as I recently had to do when applying for a semester's worth of leave. Since my state is in financial crisis, my chances of getting such a leave aren't great--and typing out a sentence that involved telling my institution how much it would cost to replace me made me devoutly grateful just to have a job.
Maybe you've done the math yourselves, with your own jobs (whether ladder or contingent), but just for kicks: what do you imagine an adjunct would get paid for teaching half my annual courseload--and/or what percentage of my six-month salary do you suppose she would earn?