Ok, but there aren't shelf scores for some fields. And many schools don't rank, and the ranks aren't finalized until the MPSE comes out in Oct of 4th year. Unfortunately, a better metric doesn't exist. If it did, no one would use Step 1 in the first place. The only reason some people do is because other than that, there is literally nothing to look at that is standardized across applicants/schools.
You can say that we need new standardized metrics across all schools, and sure that sounds good, but until that happens, this will be hurtful to students. Because students who would have been more competitive will decide to play it safe, and some will over-reach and apply and do their rotations in fields they have zero chance of matching in a few months later when their step 2 comes back.