I think some posters are quite naive about the reasons for deferring ... in an ideal world it would be purely about 'readiness for school' but this isn't always the case.
I agree that there are many reasons but I don't agree with what you say about an ideal world. I deferred Feb born DD because there is a wealth of evidence that suggests that it is beneficial for children to start school later, not just academically but socially, emotionally, etc. Also that children should learn through play at 4/5.
In my ideal world, all children would be deferred! I think, once the automatic autumn deferral comes in, there won't be many winter birthdays that DON'T defer as being a just 4.5 year old in a class with a 5.11 year old is really a big difference. So, over time, the starting age will migrate towards all children in Scotland being at least 5 by the time they start. It was interesting to see both the Lib Dems and Greens campaign on a kindergarten stage here, with school starting at 7 - the Scandinavian model has such positive outcomes, so why don't we try to adopt it?!
At 4.5 DD was writing, reading, swimming, able to get up and make herself breakfast, had a wide circle of friends - she was ready in all facets for school. Nonetheless we deferred her, as did all my 'Mum' friends with Jan/Feb babies defer theirs, and most of the Jan/Febs in her nursery. Was she bored at nursery in that extra year? Of course not!! She was reading in the reading corner, making art, growing vegetables, playing wonderful games with her friends, singing, putting on plays, visiting local woods, and engaging in all the other activities that a good nursery provides.