Last year I had a good friend come in to do an interview with students (see interview series in the applied science research blog http://appliedscienceresearch.blogspot.com) Lisa was a PhD scientist that talked about her experiences and decisions to follow that track. She initally contemplated being a doctor.
Her view on the basic difference between the MD and the PhD: She felt that MDs have an extensive database which they access on a regular basis. They collect data on a patient, and then access their database to make a diagonosis.
The PhD??? The PhD MAKES the database. There is more creativity involved.
And so it goes . . . PhDs make good role models for our research students because they are creative producers. They demonstrate the qualities we want to observe in our students. Allowing our students to work with experts promotes the cognitive apprenticeship that eventually allows them to produce the high quality product