The problem with 'red shirting' or whatever you want to call it, is that it changes the whole dynamic of what a Kindergarten class (or later grades) should be. Exceptional circumstances for red shirting make sense, but the regular use of red-shirting doesn't help anyone. When a relatively large percentage of a class (often boys) are red-shirted, this changes the average class age, and the maturity expectations of Kindergarten. This in turn gives false indications of what should be happening in a Class, from academic ability, to structured work and the length of time the children are expected to sit still without talking! It may help a single child to be one of the oldest in a class, but obviously someone has to be the youngest. I think a better solution is to expect the class to be one year group (use Malcolm Gladwell's age sorting for reading etc. maybe), but have the teachers, including those for Specials, and even Lunch time helpers, be clearly aware of who the young ones in a class are, and let them make exceptions for these children. Better this than classes we have now, with often 18 month age differences, but no differences in expectations made.
It can also make it that much harder to get the school to agree to acceleration or grade skipping, since they are afraid of creating a situation where a very young child who needs the grade skip would be put into a class of children mostly 18 to even 30 months older.
I agree that *very* selective redshirting can be beneficial for children who need a bit more time. But in my limited experience, it is more often used by uber competitive parents (whether for academic or sports reasons) or recommended by schools more worried about test scores and how an older student population gives them a higher average score.
on edit- I should have read the other posts first, since others already pointed out these issues! : )
