There are at least two ways to look at under use. @south_paw makes a great case for how some coaches exploit under use (under exposure) fears in parents to flip it into actual over use for the sake of advancing the small chance of a baseball playing career.
I look at under use within a season (Spring, Summer, Fall) as being detrimental. Things like limiting bullpens to 25-40 pitches, pitching once per week, never throwing more than 70 pitches absolutely does not condition a pitcher against potential injury. It makes injury more likely in my opinion. More than one of my basketball coaches would have us run wind sprints before our free throw practice so we could focus through the exhaustion and still hit the shots. Being late in a game, you are feeling it on the mound, you need to maintain your focus, your mechanics, keep your wits, and execute the pitches. If you have never experienced those challenges in your bullpens, are you really ready to handle it in a game? I think this is crippling to high level development if you are coddling your pitchers in practice and in bullpen sessions.
I was also a multi-sport athlete in HS, and I know that participation in each sport helped me perform better in all the other sports. I played on the golf team in the fall. The football coach came after me every summer to try to pull me into football because he knew I threw the ball well and quarterbacked in flag football leagues. I just didn’t want to get body slammed from behind on a Class B HS team (our football program was dropped to Class B two years before I got to HS). The town was bummed out to say the least. I decided I’d rather swing my golf clubs instead. I played basketball, ran track, and was on the rifle team in the winter. I played baseball in the spring.
In recreation leagues I played more basketball and volleyball at the YMCA, baseball for the town league as well as American Legion, and I was in a golf league during the summer. I also had two part time jobs. I played at a respectable level in everything I did and never was really overused. None of that mentions that I had to keep up with the homework load of college bound advanced placement courses. That may all sound like overuse, but it wasn’t. I was conditioned to perform, and I loved all of it.
Other than a partial Achilles problem from rounding a base like an idiot, some shin splints from our weekly 10 mile run, a groin pull (that happened at work slipping on a greasy tile floor), and an oblique strain suffered during a swing from my shoes that I whiffed on, I went through my teens relatively injury free. No arm injuries, and I pitched…a lot.