Agree on this.I agree with this sentiment but would expand it a bit. Any siblings, cousins or even neighborhood kids that are relatively close in age can foster competition and development. My set of twin DD's were extremely fortunate to have each other to push, challenge and support each other. I had them competing against each other in many areas since they were in diapers...so far, so good.
I have twins on my team. Good players. Last year, they convinced a neighbor of theirs who was new at our school to join our 13u team (they're a year ahead of her). This girl is not a great athlete, but a hard worker. Good kid. We taught her plenty, but the real reason she's gotten better is she's practiced/played with the twins over the past year. And they treated her like a sibling. No taking it easy on her. She learned to catch a ball mostly because they'd whip a ball at her when playing catch. They weren't being mean. They were just playing. And she adapted and learned because she had to. She's also started learning to pitch, and I'm confident she'll be able to give us decent innings at the JV level (at least) this year.
I think it's very unnecessary for kids to play organized ball year round, but if they want to get better, they really should play with friends as often as possible. Not just one sport, of course. Stay active. Do things.
(Side note: She joined our girls basketball team this year. She's not great, but she loves it. She also convinced the twins to join even though they had never played basketball before because the team was desperate for players.)