I know this is subjective (depending on the child) however I'm considering doing this, not because I think they'll need to defer, but because I am able to.
I have a 3.5yo DS and a baby DD, both their birthdays are Jan/Feb. We live in Scotland so I know this is different than children living in England.
In Scotland if the children turn 4 on their birthdays in September through to February they can start school the following August or they can defer a year. For children to defer with birthdays Sept-Dec they must apply for this giving a reason for deferring and have this accepted by the LA. They do not get funding for nursery places if they defer with birthdays in these months.
However, if they have birthdays in Jan/Feb then they have the automatic right to defer plus they receive goverment funding to attend nursery.
My DS could effectively be starting school a year in August at 4.5yo. I had not considered deferring at all as he is a quick and enthusiatic learner and extremely inquisitive with a thirst for knowledge. I sound like I'm writing his CV but I'm saying this because I think he would enjoy learning at school. To be honest I have no idea how he compares with his peers but we think he is a clever little thing 
. And we think he would enjoy that aspect of school.
I spoke to his Health Visitor about it and she said that learning is only one side of it, he would also have to be emotionally ready for it. Up until he started nursery in Feb he rarely mixed with other children as his childcare are the grandparents. He has come on leaps and bounds since starting nursery and is mixing well with other children as well as now having the confidence to go up and speak to other children when he's at the park etc now as opposed to hiding behind me!
I think in another year he could be "ready" to start school at 4.5yo. However I think we are in a very fortunate position of being able to defer him to give him that extra year before being tied to the structure of school at such a young age. That extra year will make a big difference I feel in terms of him being able to cope better in that type of environment and be completely ready to learn on that level and in that structure. I think it will also make a difference when it comes to him starting secondary school and then sitting his exams as he'll be that bit older - I mean compared to him doing the same a year earlier, not in comparison to his peers.
The reservations I had on deferring were purely selfish - it would be easier for childcare and I was concerned that people would think he wasn't bright enough etc to start school at the "usual" time, especially as I have friends whose children will be going to school at aged 4 with birthdays Sept-Dec. Therefore he will now be a year "behind" them. Another consideration was that he would be bored for that extra year and need the stimulation however he would still be attending nursery 5 days a week and I'm not sure in the grand scheme of things that should put me off when looking at the longer term benefits.
DH and I have discussed it and we are both seriously considering deferring him. We don't have to decide right now but we will before around the New Year. I would take a steer from his nursery teachers (althought the stats I have read say that only 4% of children defer based on what their nursery has said), I was also considering speaking to the school he will be attending to see their thoughts/views.
So WIBU to defer my son based on the reasons I have given? Is there anything I've not considered??