For me its a decision of whether or not to "settle". Nurses are a very important part of patient care, and honestly where I work in the ER the doctors would be completely lost without the nurses, especially the good ones. They all know a very large deal about caring for critical patients, but they dont have the experience to make the judgement calls of when to give certain meds, etc.
Nursing school is also shorter and in my opinion easier than med school. But as someone has said above, they are used for different tasks. Nurses are available to take care of patients and to do a majority of the paperwork involved in starting a patients care - med lists, allergies, etc... The docs also ask about these, but the docs are more interested in actually setting into motion the process of uncovering problems and making a diagnoses on the patient. Granted some of the older nurses can do almost everything without a doctors prompting, the final decisions still remain on the doctor, and many of the nurses are nurses because of this.
Dont know if that helped any, but there you go. My $0.02.
-Ryan