Of course a phased start helps settling in there have been various examples above where this hasn't been the case and in contrast you might say "start as you mean to go on and having a consistent routine is key to settling well".
If there was an obvious approach for a phased start that was evidenced based, I might be more inclined to consider the disruption to families acceptable. But the different approaches and logic in different schools suggests to me that it is mostly guesswork. Some schools have smaller groups for a while, some gradually increase group size, some have the whole class at the same time; some do half days, some do whole days; some free up time for home visits, some do home visits in the summer term, some don't do home visits.
I appreciate that there may be differences in what suits some children or types of intake, but this doesn't explain the different approaches taken by schools in neighbouring villages or the vast differences between the extremes of very phased starts and those doing full days from day 1.