I'm not sure hiring more adjuncts would hurt the quality of (undergraduate) teaching. In my own experience adjuncts were far more dedicated to teaching than tenured professors who focus on their research without caring how low their teaching ratings sink.
This may be superficially true, but the long-term health of the institution and the educational system are not served. Hiring underpaid adjuncts as teaching labor may allow for some good classes--either through the adjuncts' idealism or their hunger to keep their job--but it's only a sort of local maximum. Those good adjuncts came up through a system which valued teaching & knowledge. It's not clear the succeeding adjunct-taught generations will maintain this faith in their fields.
See the case study of Schlitz in the 70s (e.g. http://www.beerbusinessunplugged.com/?p=130 for a short version): desperate businesspeople go for short term gains by cannibalizing their traditional ways of making things.
