the way i understand it if you pass your USMLE step 1, you get US rotations (unless you don't want them). I've heard some chose to stay in PL all 4 years, even if they do pass USMLE.
As for the advantage... i think it comes in the form of more contacts in teaching hospitals; people who will want you there, or who can recommend you for the program you're trying to get into. IF they like you. All that helps, but your USMLE scores are still the #1 thing residency programs look at in selecting their incoming interns.