Personally, I consider the value in the visitation programs exists in the opportunity for a prospect or candidate to spend time at USNA to get a firsthand view of the school, the yard, the midshipmen, etc, so that they may make a more informed decision as to whether USNA is an environment in which they will excel.
I have seen instances in which the candidate was helped or hindered by participation in a summer program. It is not likely that it will override academic records, etc., but it could be the margin that makes a difference for one candidate over another.