D.F.
I was saying the very same thing when my son was at the age of entering kindergarten with a Aug 29th birthday. Ready achedemically but not socially. So I held him back and did 4 yr. old preschool again (at a different school) well he is now in 5th grade. It turned out when he did get to school he caught on very quickly the teachers even had to give him extra work to keep him busy. Also, socially he did a 360 his shyness/mommas boy thing just dissappeared after a couple weeks in kinder and he became Mr. personality. When he realized I held him back (in 2nd grade), he would bring it up and let me know he was not happy about it on several occasions. I think because school was not difficult for him he enjoys it more than if he had to work hard at it. Then....
When my daughter was 4 with an Aug 15 birthday and was doing great in preschool. I made the choice to send her to kinder, the choice seemed much easier. But by 1st grade she was not reading well and school was more of a struggle for her. I think she might have a slight case of Dyslexia but the teachers say no. She is now in 4th grade and has to work harder to keep up, I can only think that holding her back would have given her more of a head start.
The thing is at that age it is so hard to determine what will happen when they get into school. I was a SAHM mom and keeping them home an extra year was an easy choice. Once they get in school holding them back a grade is difficult in many ways. In my opinion you can't go wrong keeping them home 1 more year.