The socialization aspect of a PG needs to be considered. By senior year, most students have already made their friends circle at boarding school. Its hard for PGs to be accepted by the others (who have been there for 3-4 years). The PG’s mostly stick with each other.
My son’s school had PG students. He knew a few of them, but wasn’t very close. The PG students boarded in the same dorm as the seniors.
For college readiness, the PG year is basically a repeat of 12th grade. It also won’t necessarily help with college admissions either.