Anyone watching Taishou Yakyuu Musume 01 saw how girls were shocked at the violence of the sport after a player was knocked out with a ball. Also, I recall some bloggers mentioning anachronistic aluminum bats in TYM, although I didn't watch far enough to see them myself. Now, check out this:

In 2003, an 18-year-old pitcher in an American Legion baseball game, Brandon Patch, was struck on the temple by a batted ball and died only hours later. Attorneys for the mother of Brandon, Debbie Patch, filed a civil suit in Helena District Court seeking damages from the makers of Louisville Slugger bats, claiming that aluminum bats are too dangerous.

Since this is not a show by GAINAX, hopefully Akiko is not getting her brains bashed out in ep.6 thus allowing Koume to flourish.

UPDATE: Evirus comments:

That is a big reason aluminum bats haven't made it into the big leagues; at a professional level they would kill pitchers.

Also the whole changing the balance of the game thing.

They're still used at the collegiate level because it's cheaper than replacing wooden bats over and over, but the use is also blamed as the reason many pitchers don't know how to pitch inside (throwing the ball very near the batter) anymore--it's not as difficult to hit an inside pitch with an aluminum bat, and nearly all pitchers now will never face a quality batter using a wooden bat until they are already professional.

I knew I'd learn a lot from TYM, even if indirectly.