This is one of those things that might sound good up front, but end up being more detrimental in the end.
MOST kids at a young age don't really hit hard enough to cause that kind of damage IMO.
But! They are learning the proper ways to tackle and angles and head placement, etc, etc
Now, when these kids start playing ball as decent-sized (and don't forget about some huge size differences) 14-year-olds, I can almost guarantee it will lead to more injuries. Not all will be brain/head injuries, some will be debilitating neck injuries due to the lack of knowledge and proper technique that comes with playing the game of football.