Hal, why aren't there any "famous" pitchers doing this? I watch the college world series every year and have never seen anyone switch between submarine and windmill.
I am not Hal, but one of the reasons that girls don't use it is that there are very few of us that can still teach it.
Twice, I have tried to teach this to girls that were tall, gangly girls that just weren't getting the arm circle. One didn't continue pitching and one's dad didn't want her to be different. She already was different, so he didn't want to exasperate her problems. She actually could throw the slingshot pretty well, the windmill - not so much.
I find the slingshot to be less stressful on the shoulder, and back in the 60s, our 1 pitcher would throw all weekend, and I can't remember any arm problems.
Hal, my youngest DD 12 yo can and practices it often, it usually freezes the batter, and sometimes has an uninformed coach calling time to question the blue...She can do her normal double pump, pitch right from the hip single pump style and submarine style...