I thought about this as well.
I had a coach of my sons yell at me, literally yell at me, when I brought up weighted baseballs. He said they lead to injury. I was familiar enough with some other training stuff, track and field for example, to know that overload/underload training is common in other sports. He followed up yelling at me by suggesting he throw a football for arm strength. This made zero sense of course. He just knew that QBs don’t get injured like pitchers do. He didn’t know why. I thought about it and I finally realized it is not the weight of the ball…an average football weights three times that of a baseball…it is the arm speed. Because the football weighs more the arm cannot move as quickly. It is the arm speed that is dangerous if someone is not well prepared.
It makes sense in a way. The majority of injuries in pitchers occur at or after ball release…the shoulder, back, forearm and other physical mechanisms are struggling to slow the arm. This is why the risk of injury is generally higher for a guy throwing 90 + as opposed to a guy throwing 80…and it escalates pretty steeply.
This is why throwing underweight baseballs is way more dangerous than over weight balls.
So, weighted balls are good for a couple of things. They can be used to correct bad throwing patterns. Adaptation is a key component of any training. The body has to learn to move more efficiently as it is challenged. If someone is doing body weight squats for example there is a lot of room for bad movement patters. When they start putting weight on a bar, then heavy weight on a bar, their body is forced to learn to move efficiently through the lift in order to execute it.
The weight of a baseball, 5 oz, is random. There is no magical reason it is 5 oz. So, throwing a 7 oz baseball is really no more inherently dangerous than throwing a 5 oz ball. Your body is already used to a 5 oz ball.
Just like any training guys get into trouble by not understanding what to do, jumping in way too fast way too soon or training badly.
Weighted balls are not a magic bullet and they are not a toy. No long toss with a weighted ball, no throwing off of a mound with a weight ball. There needs to be a good foundation of health and a good throwing foundation before any weighted ball program should be attacked.
I believe that weighed balls need to be thrown balls out, so to speak. Thrown hard to maximize results. This is why a good on ramping program is key.
Driveline Baseball has a very good free program that is 12 or so weeks long.