Hi Amy,
In the healthcare system I work for in NC, it is almost unheard of for there to be an "outside hire" for a full-time position. In my experience and that of a lot of my CEP coworkers, most started as interns while finishing up bachelors/masters. Upon graduation, we applied and were hired as PRN employees, and then in a FIFO sort of manner, as full-time became available the position was offered to the most "senior" PRN employee.
Applying for positions is highly competitive due to our proximity to a state university that graduates roughly 100+ bachelors students and around 10 masters students yearly. That doesn't take into account the other colleges in the state that also vie after positions in the city.
It definitely seems to be a supply and demand situation and PRN or per diem positions are a cost effective way on the healthcare system to hire extra labor without having to pay out benefits. Coming from the experience of being PRN, it does offer the employee flexibility and if their manager is willing to give them set hours, it can be a relationship that benefits both parties.