What works best for one kid might not work for another kid. What works best for one school might not work for another. There are a lot of factors involved in the decision to move an 8th or 9th grader up to varsity, I have been through this with my kids.
need at the position
school policy and or philosophy
each case is different and you have to do what is in the best interest of your kid and hope the varsity coach does what he feels is in the best interest of the kid but his responsibility is to the team and the program as well as the individual.
If a 9th grader is the second best goalie or second best fogo in the school it might be in the best interest of the kid to play JV and get the game experience but it would definitely be in the teams best interest to be on the bench for the varsity in case he is needed so you have a clear conflict between what is better for the kid and what is better for the team.
There are opinions but know right answers