The argument is that the NPs do not have the education and training that MDs have. As a nurse, I have to say that nurses are trained to see the entire patient from the start of their RN program. We learn assessment skills and have the responsibility to know what medication we are administering and as RNs we have the right to refuse giving a medication, or to question the order. Physicians spend medical school and internships rotating through specialty areas as well as primary care. The NPs that I know who are in primary practice typically take a great amount of time with their patients as well as seeing the total patient. A nurse practitioner is also going to refer a patient to a specialist (as primary care physicians do) if they see something or suspect the need for further follow up.
The need for independent NPs is threefold. The baby boomers are aging and are going to need greater access to healthcare. On the opposite end, The Affordable Care Act is going to bring a flood of newly insured patients into healthcare. Finally more physicians are going into specialty areas due to Primary Care being one of the lowest paying specialties. It is my very biased opinion that Nurse Practitioners have the ability, skill and knowledge to be excellent primary care practitioners.