When I coached, I pitched batting practice from the circle. Up until 14u I trusted myself to field the balls coming back through the middle. At 16u the girls were stronger, the balls came much faster, and I was getting older . I didn't wear a mask, but I refused to pitch without a net. Even then sometimes I would get hit in the leg because I landed outside the net's coverage.Not excusing the behavior but it was my understanding that the pitching distance in fastpitch is smaller plus the bats are hotter. When you combine those two things you really should be wearing a mask. Anyone who thinks otherwise should stand on the pitcher's plate and let me hit ground balls at them with a fastpitch bat.
It has nothing to do with technique and practice, it's about physics.
DD was a pitcher and we required her to wear a mask until she was 18. Required may be a harsh term as she never even asked to not wear it. She also wore it when playing at a corner infield position. She continued to wear it through her freshman year of college. She stopped wearing it at that point. We discussed it, even though she was 20, but ultimately we made it her decision.