No doubt "overuse" is the main reason for injury. That problem exists in one form or another in almost every sport from marathon runners, to gymnist growth plate issues, cyclist with hip problems, swimmers with shoulder issues, etc.
Couple points that I've noticed in my limited tenure as a ball coach at the youth levels:
1 - Strike zones are "loose" for the younger ages. Coaches know this. So, when they observe an umpire with a loose zone, they will throw the kid with the breaking ball more frequently. We played everywhere from East Cobb to Florida and most umps at 11U are giving the pitchers at least 6 inches on both sides of the plate.
2 - Back to the original poster's point: "Stupid Coaches" is kinda harsh, but I think he's referring only to those that have no regard for a players wellfare. If a kid has a breaking ball that he can throw for a strike, the coach is likely to call that pitch. And he's likely to call that pitch more often. And if the pitcher is having success in a tournament game, the coach is not likely to pull him until the kid is tired. That's the primary reason I don't want my own kid throwing the breaking ball. If a coach knows he has it, the coach is going to use it frequently. And likely use it until the kid gets fatigued. Just came from a WS tournament in Florida where the tournament winning team pitched their breaking ball throwing 11U pitcher 16 innings over 3 days. Proper curve ball techniques don't matter when an 11 year old kid has to pitch that much. Its too much. I think its these type coaches that the original poster was referring to.
So yes, throwing too much is the main reason for injury, but having a successful breaking ball at a young age might actually worsen the former issue if the team has a "stupid coach"...