Both my boys have summer birthdays- June and July- and we didn't "hold them back."
Fortunately for us DH was able to work night shift in the AF and I worked days when they were little so they never had to go to day care. This allowed for a lot of time for us to teach them at home, and they were reading by age 4. But we were concerned with social skills and adjusting to a classroom setting.
Schools here offer a 4 year old pre-school program and kids get screened- the ones who "test" lower get in and there are 20 spots for morning and 20 for afternoon classes. This is to ensure that every kid is ready academically to start kindergarten. Both boys didn't get in based on this, but because the 40 spots didn't get filled we were able to get them in anyway. It was a great transition for them.
Now at 14 and 18 I still don't regret not holding them back. We ended up moving and changing school districts when they were starting K and 4th because we felt the classes weren't challenging enough (the oldest and several of his classmates were getting sent to the 2nd and 3rd grade teachers for math and English during 1st and 2nd grade, then they were repeating the same things in 3rd) and have been very pleased. Although neither are super academically gifted they are still learning more and not bored.
From a maturity perspective I never would have thought to hold back our oldest. In contrast our youngest can be immature at times compared to some of his peers, but it's just an opportunity for us as parents to teach him how to behave properly. That's why I understand if parents decide for this reason to hold their kids back- each parent knows their own kids best.
From a sports perspective I don't think it's fair, but it's also not right to penalize a kid who may be bigger and has not matured in skills because of their age. Our youngest has always been one of the smallest on his little league team- and they base it on age not school grade. He's had plenty of kids who are a year younger be bigger than him. They weren't the best players on the team though. They can play on the school team in 7th an 8th grade. When he went to try out he knew right away he wouldn't make it after seeing some of the huge kids there- he was like "The coaches aren't even going to give me a chance!" He still tried out and we were proud of him for that. Thankfully he's still had fun playing little league and is mature enough to realize he's not going to be a professional athlete and that's ok.
OK, sorry for being so long-winded! I guess my point is that each parent needs to make the best decision for their kid, and even if others don't agree with it, I think each parent in their own mind does put their kid's best interest at heart. Do what works for your family!