The thing is this. It's easy for us guys who may have played the game in some form to say it's easy to get out of the way of a line drive, or even catch it. But I would say 50% of guys and 80% of women and children will not. And these are people that are paying attention.
The other disconnect I don't think people understand is this. Most balls that go into the stands are lack of a better word foul tips. Of the thousands of foul tips that go into the stadium, very few fans get injured. And probably most of the injuries occur from fans fighting for the ball.
Of the other types of things to go into the stands are line drives, bats, and players. Which compared to foul tips are a very small %, maybe a a few game if the fans are lucky, and mostly down the line where you can react. It's those that are behind the dugout where injuries probably occur most. And as a %, you may have an injury rate of less than 1% for foul tips and even line drives down the line. But you go that dugout area, and the line drive injury rate probably skyrockets. The screens would help mitigate that other types of injuries.