If I may add my two cents...while summer practicing sounds good in theory, reality says otherwise. After an entire off-(and pre-)season of individual and team workouts, coupled with a pretty intensive spring high school schedule, the last thing that these players care to do is sit out on a practice field during the summer. Right or wrong, like it or not, their minds are just not into it which, more times than not, results in a rather meaningless practice session.
Instead, and seeing that our baseball season is as short as it is, I would much rather my son see as much pitching and game situations as possible during the summer months and allow for those "teaching moments" as Playball2 alluded to. At the same time, the more serious players will still find time to put in whatever individual training is necessary along the way. As far as I'm concerned, play as many games as you can during the summer and spend the winter months honing the mechanics...JMO.
And, by the way, my opinion pertains to travel, high school, and legion baseball equally across the board.