is most kids spinning a ball can't locate their fastball accurately. Why spin if you can't locate?
Also, high velocity youngsters are at higher risk of UCL injury than lower velocity youngsters. With my son throwing the top of his age in velocity, I don't let him spin.
Curves at 12u and 13u don't tunnel, so what's the point? It is just a velocity change. That is something that can be done with a change up the safest pitch in baseball.
I already knew that thrown a curve hasn't been proven to cause injury, but I was under the impression that it hadn't been proven to be safe, either. I'll check out tbag study.