Whilst I agree that for many summer borns they suffer no disadvantage (3 of DDs summer born friends are sitting 11+ exams this month), for some it is a very different story.
DD2 had turned 4 the week before she started reception, against the recommendation of her nursery, who approached her new school. Whilst the school were in full agreement with her nursery they could only offer to defer her for a term.
Roll on a few years, she's just started yr6 and we are still waiting for the anecdotal 'they eventually catch up' moment. Have spent the past few years hearing this from her teachers, and 'Well,she is the youngest in her year'. It has become very clear that the gaps created by starting reception with such immaturity have just widened. The building blocks her classmates were able to absorb in the first few years were lost on DD, it took enough of her concentration to sit in one place and listen. She is always a step behind, leaving her de-motivated and detached from learning. Her only saving Grace is her amazingly resilience and great sense of fun and adventure, ironically maybe strengthened by the struggles she has faced)
We didn't want her to start reception a year later to give her an advantage over other kids, we wanted her to at least have the chance to reach some of her potential. DDs end of yr5 report was the first she had ever had which indicated she is heading toward average age reading and literacy levels, for which we are hugely proud. But what she might have achieved had she been ready to absorb all the learning available in the foundation stage.
We are already having sleepless nights, worried about her starting high school having just turned 11.
It is for kids like this that there does need to be more flexibility and fluidity. failing that, some real resources put back into schools to support those children who have had a bumpy start to their education.