A blogger named Michael Kennedy wrote:
Medical school teaching was far better in the days when full time faculties were small at most medical schools and a lot of the teaching was done by unpaid volunteers.
In my medical school, U of Southern California, there was a tradition of voluntary faculty in surgery. The chief from 1932 to 1969, CJ Berne, was part time and donated his salary to the school, as did the chief of Pathology, Hugh Edmondson who was full time but wealthy. The surgery department had three full time members at the time I went through training.
I also spent six months at the Mass General as a medical student and most of the teaching there was by the likes of Claude Welch and Marsh Bartlett, both of whom were full time practitioners in the private wards.
Now the USC surgery department has 90 full time members and, about 20 years ago, the part time people were told they were no longer needed.
Interestingly enough, during my era at USC only one graduate failed the boards and he was never allowed to forget it. About ten years ago, they had about one-third failing the boards on the first try
Our teacher prep program has a number of fantastic part-time instructors. They tend to excel at gritty, real-life issues -- where theory breaks down and teachers are left with messy what-do-I-do-now situations.
A number of charter schools, including ours, use part-timers, too. Typically these are former full-timers who become moms and want a lifestyle change.
Our experience has been that part-time teacher is a difficult task, because it's so hard to contain the kids' needs. But part-time special projects coordinator or part-time teacher/tutor coach works fairly well.