Well honestly I've never used the flat sided ball you're referring to, so I can't make a comparison there. But this PC I spoke about earlier was only the second PC my DD has ever had, the first one was a older guy from age 10 through 16. This new coach was a woman on her mid to late twenties, she introduced the spinner to us, I have never hated one thing ever in my life as much as that hard yellow spinner!!Fair enough. I'd be curious why you think the spinner is better than the ball with the sides collapsed.
After warming up, she would put Anna through all her pitching drills with the spinner first, 12 to 15 minutes worth, it took probably 3 maybe 4 lessons for her to get really proficient with the spinner (and for me to stop hating it) and it really tightened up her spin with EVERY pitch, but especially her curve and riseball. If not for this instructor, I would have agreed with you 100%, I never had any intentions of ever using one! But......I saw first hand when properly used, it's a very effective tool, at least for us! She made every one of her students use one, from 10 year old beginners to college pitchers.
BTW, this older PC I was talking about was from Akron Ohio, he played in a men's fastpitch league, Anna was probably 12 years old at the time, so roughly 8 years ago? At one of his lessons your name got brought up, he knew you, he said he was supposed to pitch against you one night in a game many years ago, and that some kind of crazy depression came through (terrible storm) and it was cancelled and never rescheduled. This guy is in his mid to late seventies now and one of the most incredible human beings I've ever had the pleasure of being around! It took Anna quite some time to adjust from being with him for so long to going to this new PC, cause she's TOUGH! You don't do things the way she likes, she loves "activities" which is her term for running, burpees, push ups, and sit ups. She had a really good way of helping a girl focus, I loved her!
Anyway, that's my spinner story!