In my experience, every child benefits from a few half days in a small group to bed in. Those who don't wish for any half days at all are looking at what benefits them as a parent, rather than their child. I'll explain why I firmly believe it benefits all.
All children are different, and there are of course some children who are definitely ready to start school. There are many many others these days who are not school ready. If they all start full time from day one, the ones who are ready do ok. They have a nice time playing with the continuous provision etc. They also tend to get very limited adult attention in that first week or so though though, as many Reception classes operate with just two adults (teacher and TA). Their attention is pulled many different ways when they have 30 children starting together for the first time ever. 30 children all coming in who have no idea where to put their coat and bag, and what to do next. That's ok if they all come in calmly, but in reality there will be screamers. The staff are juggling peeling screamers from parents (which can be quite unsettling for even the most confident of children, if they don't know the child who is screaming and don't know where to put their own belongings, and the adults are unable to support them immediately as they are needed elsewhere).
When they're all full time from day one, the children can often have very disrupted carpet sessions for the first few weeks, while the adults are coaxing children who are refusing to come to the carpet to join in, and other children are being bounced on someone's knee while they sob all the way through the session. It's not the best way start, when in an ideal world you want carpet time to be calm to start as you mean to go on in terms of bedding in rules and expectations. Of course, disruption can still happen in small groups, but there are generally far less children to juggle who might be struggling to adapt at any one given time. Often these can be dealt with by just the teacher or TA, while the other focuses on the rest (swapping throughout to build relationships with all). When they're all together, with the best will in the world, the children who are 'fine' to be full time, often get left to their own devices during those first few days, while the adults juggle having so many unsettled children all at once. Staff feel so guilty about it, but it's the reality.
It's so much easier for young children to pick up new rules and routines in a small group. Sometimes the TA may need to take a couple of children off for a walk so that the rest can hear the teacher and learn the basics. They then become the role models for the rest, which in turn helps their peers to settle if they come back to a room of children who are all engaged and modelling what to do. When they're all together and it's too many unsettled children for one TA to whisk off, it becomes a juggling act between teacher and TA while other children are a bit bewildered what they need to do. She's saying to sit here and join in but that little boy is running off over there and that girl is screaming rather than turning her voice off and now I can't hear the teacher and I don't really know what I'm meant to be doing. You get the gist.
So while some children might be 'ok' to do full time from the off, their experience can be made ten times better in a small group for a few days, until they know what they're doing and have started to bond with the adults etc.
I find it so sad that, at a time when we're hearing more and more reports that large numbers of 4 year olds are not school ready (which is true, I'm living the reality every September), more and more people seem to be pushing for Reception children to start full time from day one. Staggered starts have always been about what is best for the children. We also have phonics schemes all pressuring teachers that they must start the sessions from day one in order to complete the expected coverage by the end of Reception etc. It's all too much for these little people. At the same time, teachers are also juggling the pointless government Reception baseline, where we have to sit 1-1 with each child while the poor TA juggles the rest of them. This is all the more chaotic when 30 children start full time together. Yet more and more schools are now bowing down to the demand for full time from the off, to avoid complaints and being accused of violating an admissions code ( 🙄 ) which is not in the best interests of the children. It's so sad.