Someone will correct me if I am wrong, but I am pretty sure it is more of a help for your older players to help maintain their skills longer (less ratings drop each year), with an additional effect of reducing the overall chance of an injury to all players. Coaching ratings are the biggest variable to young players developing (aside from proper use of minors, promotions, etc). Also I think I read somewhere in an old post that a players makeup effects both injury recover, skills development, and speed at which they develop. Of course playing time is the big one for development.