The way it works now: a new player entering Rated for the first time will face immediately at least semi-skilled people and be completely obliterated; if everything was Rated then the new player would first enter a BG with new or bad players; only if they managed to be great would they manage to escape that rating (and premades would cause no serious issues since they would easily jump rank too and it could be an account-wide thing too).
Perhaps it would even solve the issue of Arena being morbidly hard for new players; nobody would be able to enter an "easy mode"; but that in turn would mean the great players would be completely gone from the arena of a new player so the new player would face either new players or bad players when they first enter arena (given it's not the first hour of the season at least).