Interesting topic and there is not one "right answer" for this one.
None of ours are skipped (nor do I think they should be...not PG). You might call middle kid a red-shirt. When we changed schools we had her repeat 2nd for social/maturity reasons, though she is now in the correct grade for her birthdate (private school had a different birthday cutoff).
I think it may be tough to know if you made the right decision until well after the fact, perhaps not until they go to college. And sometimes you may not know even then...you can't run two scenarios at once with one kid. You'll never know if they would have done better or worse with academic competitions, athletics, etc. if you skip or red-shirt.
With that said, I think that the middle kid outcome is mixed. As far as academics go, probably would have been better for her to go to 3rd in the new school rather than 2nd. However, she knew a lot of kids in 2nd through rec sports. Also, my kids are small and not naturally athletic, so that extra year helped a lot for middle kid. If she was a year ahead, I doubt she would still be playing sports, but she is now in 10th and plays a fall sport and a spring sport...she was really excited to be the only 10th grader to make varsity in her fall sport.
While some may not find sports to be valuable, my kids have gained a lot of good memories and friendships through sports. My kids acknowledge that school has been boring at times but I doubt they would have wanted to skip...but who knows what the outcome would have been if they had skipped.
