It is not a bad idea. Batters are getting stronger and hitting the ball harder at all levels. I'd allow it and leave it up to the individuals to wear one if they like. I have seen people get hit in the indoor nets and it'd make sense to protect their heads.
This!
Once they become more readily available I'm wondering what the stance would be.
Even 15 years ago colts wearing a helmet to bat or stand up when keeping wasn't compulsory, but is now a requirement for anyone under 18.
The knock on effect of this is rather interesting, as there are people only a few years older than me who don't wear a lid ("never used one when I was younger, why start now?") while almost everyone age 18-25 choses to wear one as they always had to while growing up/learning the game.
Will these also become a requirement for under 18s in the future? If so they could become common at most levels in just a few years.