I appreciate the caveat you put on there, as I HATE this adage. To believe "playing against tougher teams will make you better" is living with blinders on.
Playing competitively against tougher teams will make you better.
Too many lower level coaches who aren't good at their job but believe that they just need to go get curb stomped on a weekly basis and they will magically get better.
Totally agree. This isn't basketball. Individuals may perform better somewhat as the season continues, but overall they're just using whatever level the brought in from the offseason and trying to hold onto it. That especially applies to pitchers.
Really the only thing I see kids get better at throughout the season is communication and learning to avoid mental mistakes, and things that have nothing to do with their individual skillset. There are just too many games to expect much improvement to happen in those 1-2 practices they manage to get per week during the season. This is why hold two practices a week year round. Getting many of them to show up is another challenge entirely.