In all the many years girls have been playing softball, of all the millions of pitches thrown, how many pitchers have suffered truamatic head injuries that would have been prevented by a mask?
Please cite your source(s).
I too have been around a good many games over the years and have never seen an injury that a mask would have prevented.
Even so, a few years back my DD tried a mask. She said it was uncomfortable, hot, sweaty, would slide on the sweat, impair her vision of the ball, get tangled in her hair, was dirty, caused her to break out, etc, in short it distracted her mentally and hindered her performance.
Do I worry? Not any more than when she's surfing, snowboarding, jet skiing, sky diving, hiking through Death Valley, snorkling or scuba diving.
Edited to remove a statement that would be spun endlessly by the vociferous velociraptors.

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