If it is to bring kids up so other schools don't get them then I think that is wrong. But some on this thread seemed to be stating it was either all or nothing. I just believe it depends on the individual kid. There shouldn't be a rule of thought that all MS are not ready for HS ball or that they should only be allowed to play freshman and JV.
It is a valid point that if a MS player is not strong she can be hurt by an older, stronger player. However you can be a small 9th or 10th grader and hurt by a bigger stronger senior as well.