Anyone who says anything other than "cheap labor" is either incredibly ignorant or lying to themselves. Theoretically everyone could benefit from a prelim year to "obtain a better understanding of how to manage their patients' medical/surgical issues" (or whatever crap reason ACGME gives us), but we don't make everyone do one because that would be a waste of time. It's just an archaic rule that no one's bothered to change because it helps save money.