Lots of good points made here from both sides. I can only speak for one, single individual. My son played every sport possible through age 10. He played baseball, football, basketball, golf and even tried some tennis. In middle school, he played baseball, football and basketball. We did play some fall baseball and yes he did pitch. There were extended periods of time where he did not pitch or even throw a baseball. I can't give you specifics because I didn't keep track of this info. I did coach him through age 12 so I am acutely aware that he was not overused or abused as a pitcher. We set pitch counts when the rules actually went by innings pitched. I thought that was a stupid way to "protect" a pitcher. He didn't throw breaking pitches until he was around 14 or so. He played basketball and baseball in 9th grade. He loves basketball, but the coaching was horrible so he decided he didn't want to play basketball after 9th grade.
At 16, he felt a "pop" in his elbow while making a throw from the outfield during a try out for USA Baseball. He was diagnosed with a UCL tear, but Dr. Andrews felt that the injury could be treated non-surgically. After 5 months of rehab, he returned for his sophomore baseball season and played without incident for the next 2 years. He played spring baseball and rolled right into summer baseball and also played fall baseball. My best estimate is that he shut down November and December. I am not saying he didn't throw a baseball at all during that time period, but he wasn't on a throwing program during that time and he certainly did not pitch at all. He would continue to hit and probably would throw a little indoors here and there just goofing around.
In the middle of his senior HS season, he threw a complete game which consisted of 89 pitches. It was his best outing of the year. He woke up the next morning and told me he thought he had torn his UCL again. I didn't see how that was possible because he had no complaints of pain or soreness when he went to bed the night before. His team trainer saw him a couple days later and felt like it was just soreness. No joint mobility or weakness was noted. After a couple weeks, he still couldn't throw without feeling a "sensation" in his elbow. We saw a sports ortho that found nothing wrong. I insisted on an MRI and the doc finally gave in, begrudgingly. At that time my son was being given serious consideration for the draft and I wanted to make sure there was no structural damage and I knew this would be an issue with any team interested in him. The MRI was read as normal. As a matter of fact, they said the ligament looked much better than it did in the MRIs that were done 2 years prior. We decided to shut him down anyway and he only DH'd for his HS team the rest of the year. So from April 10 until he arrived on campus for college and started their throwing program under supervision by their trainers in July, he didn't touch a baseball. His throwing progressed without any issues at all. His long toss and flat ground went smooth. His first bullpen from the mound, he felt the "sensation" again. He was diagnosed with ulnar nerve entrapment and had ulnar nerve transposition in mid August. It caused him to miss his first fall practice, but he rehabbed hard and got all the way back to mound work a second time by early December. No pain or issues at all. On the 5th pitch off the mound of his first pen, he felt a "pop" with pain. This was the first time he had felt any pain with the exception of the tear at 16. A MRI confirmed the tear and he had surgery on December 16.
I'm sorry for the length of this post, but I just wanted to give one person's personal journey and say there really is no rhyme or reason why some guys get injured and some don't. Obviously overuse and those things increase your chances of injury. Trust me, I beat myself up the first time he was injured, but Dr. Andrews put me at ease. He explained that the elbow cannot withstand much over 85mph. So all the high velo guys are just one pitch from injury. He said if doing the "right" thing was all that was necessary, then why do we have MLB guys getting injured? They have access to the best information and best training modalities. It could simply be structural issues. Anyway, I'm not saying I don't agree with all the above posts. I just don't think it is as simple as take 3 months off every year and don't throw over 100 pitches per outing and you will be fine. The pitching motion is such a complex and un-natural movement. As long as athletes push their body to maximize output, there will be areas that break down.