Junior (10 starts as a junior this past spring) pitches the final two innings of a meaningless HS summer game last week. Coach wanted him to get some work in. Throws one pitch, strains a muscle in the rotator cuff and now he is shelved for the rest of the summer. Ouch.
So, my experience would lead me to say yes, HS summer ball hurt his development.
Also, Junior played s****r his freshman and sophomore year, but did not fall of junior year. Even though he worked out, his conditioning was not at the same level as it had been playing s****r.