I haven't read all the responses yet. I'll go back and do that in a minute. I do live somewhere where red-shirting is the norm. My son with an April birthday (so 5 months before the Sept.1 cutoff) was the only child I knew born that month who was sent to school on time. Boys are routinely enrolled a year later here. It's pretty common for kindergartners to be turning 7 by the end of the year.
I eventually, not only sent my son on time, but he ended up skipping 1st grade. It means that his acceleration was a bit more radical then we planned on it being as he's a whole 2 years younger than many of his classmates. It's been interesting, but nothing that would convince me we made a poor decision.
I'm not a proponent of red-shirting. When I did my research on early entrance, grade skipping, ect. I encountered a lot of research about late entrance and retention of students. Most of the research shows no advantage to holding kids back. There can be some early advantages in early grades, but those in large part disappear by 3rd grade. There are even studies that show poor outcomes for students who are older in high school, including higher drop out rates.
So over all I think the practice has very little good going for it despite the dozens of anecdotal evidence everyone in my town regurgitates. However, I do see the appeal. Red-shirting is so common here. So the kids get to kindergarten a year older and are ready for more advanced school work. The teacher finds herself teaching more advanced concepts in kindergarten and soon kindergarten is much more academic. Next thing you know it's too academic and a poor fit for a child sent on time. When you see what's being taught and can't imagine your child being ready, of course you hold them back.
I think the answer is to hold our schools accountable, to move able and ready students forward, and to ensure that our teachers are prepared for our sent on time kids.
As a parent who has made unconventional educational decisions for my own child I cannot in good conciseness say that another parent shouldn't do the same. If waiting a year is what feels right to to that parent for that particular child then I support them in doing what they need to do.