Yea using nursing as a "backup" is a bad idea. As is getting a BSN degree and going straight to medical school. Those who have successfully transitioned from nursing to medicine worked as an RN for at least 6 months. Working as an RN you gain tons of experience. You get to witness first hand the various roles of health care providers (MD/DO, Pharmacy, PT, OT, ARNP, PA, etc etc). As an RN you also build relationships with Physicians. It makes receiving guidance, LOR, and acquiring shadowing opportunities a breeze.
To use myself as an example, I work in the Neurological ICU as an RN. Ive worked directly with fresh post op Craniotomies, pts with ventriculostomies, SDH, GB, MG, vented/intubated pts, etc etc. Ive been in countless amounts of Code blues and stroke calls. I have a close relationship with the neurosurgeons, anesthesiologist, neurologists, neuro-interventionalist, neuro radiologists, hospitalists, cardiologists, etc etc.
That is how nursing can be beneficial form someone pursuing a career in medicine. But like others have stated, you really need to have a solid reason as why the switch. If you do then you are golden. Although I have no regrets since the experience I received was sound, nursing is not for me. I learned that through my experience as working as one. Its hard to say nursing is not for you if you never worked as one. So obtaining a BSN and never working as one will hurt you when you apply to medical school.