This is a compelling question and very interesting to me. As people leave the hospital sicker and sicker AND people live longer aided by high medical technology, the issue gets more and more convoluted.
Almost buffet style, we have seen skills in the other professions that we need; likewise other professions have feasted on our skills (and even done some "take out dining"). So professions have found themselves sharing technologies, skills, approaches. However, the true domain of the nurse is hands on, bedside nursing. I'm not trying to knock those of us who aren't bedside caregivers, but that is what the core competencies of nursing are based on. And the role has grown.
What I saw happening when I did case management for tech dependent children is that we had high tech, even vent dependent children, and we taught parents to become very competent caregivers to their children. And they made complex decisions about their kids frequently (problem solving vent problems, plugged trache tubes (changing them out asap with aplomb that would have impressed hardened ICU nurses), etc, etc. This is what we do when we teach and delegate med admin to CMA's in care homes; Accu-check or vent dependent child care OR g-tube feeding to a teacher or para or school secretary; or vaccination administration to a Medical Assistant.
We have seen ourselves delegate these kinds of major tasks because of the proliferation of the chronically ill living longer and in the community and in chronic care facilities; because nurse need exceeds demand; and because we have become costly employees.
But I still feel that when conditions change rapidly (physical instability) or when the situation changes in an unanticipated way (a diabetic child becomes sick at school and what their accu-checks are doing aren't described in usual procedures) NO ONE is better at directing the nursing care than a real live Registered Nurse because we have the comprehensive body of knowledge that allows us to accomodate to change, provide supportive or other care and change directions. This care may include, naturally, involving a physician.
Much like the Nurse Practitioner in a doc's practice, nurse extenders (CMA's, NA's, LPN's, para's, teachers acting according to a health protocol, my parents caring for their child in a high tech situaion) can probably do WELL ENOUGH until change, unstability or a new rare or new situation occurs. THen you need a real nurse. I'm not trying to dis physician extenders, but we can acknowledge that their knowledge base is not as comprehensive as docs; just as "nurse extender's" knowledge base is not as comprehensive as RN's or NP's.
Our knowledge base simply qualifies us to do a better job of problem solving changeable or complex care situations. To me, that's the difference thought not highly quantifiable.