"I do take the point about the local children though. The school is in a very affluent area so they probably could easily afford private education."
Well, no, very few people can afford private education and very few people can afford to move to an affluent area to be within a catchment. I live in an affluent area (by sheet good luck) but could neither afford nor want to send my dc to private school!
But what very often happens, as has been described several times on this thread, is that a family will rent a flat or house within a good school catchment for a year, get the first child in, then move to a bigger house in a more affordable area, whilst continuing to send their first child and all the rest of the family to the school.
Many families at my local school with the tiny catchment do not live within the catchment but did at the crucial time when they were applying for their first child.
If secondaries did not have a siblings policy then probably more local children would get a place at small catchment schools.