1. It would put the university in a better moral position. Exploitation of the people who work for you, the people who bring in the tenure dollars you operate on, is immoral.
2. It would make it less profitable to teach up to 86% (History) or 69% (English) or 80% (Humanities) of your classes with adjunct professors. If it is less profitable to do so, then it's easier to think about how to hire more full-time professors, even in hard economic times.
3. Absent the cash cow that adjuncts are, we would think better about more fully staffing departments. A larger set of full-timers leads to a better sharing of committee work and to better teaching.
Forrest Williams liked the idea, and added the thought that if department chairs all refused to open new sections with adjuncts, announcing as they did so that it was in the service of building good departments to serve students who are themselves being cheated by departments without sufficient full-time faculty to pay good attention to them, we would send a powerful signal.