My kids were pitchers and I watched all this summer high school baseball for the last 5 years. My opinions of it are mixed.
It is better to face hitters than just throw a bullpen.
"Throwing" versus "pitching" doesn't develop the pitcher's pitching skills.
Our pitchers never call their own pitches during the spring but call their own games in the summer (other than the playoff games.)
However, if a kid has mechanical issues it generally is not going to be worked on in the summer.
Looking back I would say it was better than nothing but at least at our high school, it really did not help them make big strides in developing their pitching skills. As for exposure, I see fewer and fewer people coming to watch.