"mini
The protection against PND is surely only applicable to women who want to bf. "
Why should it be?
"Failure to establish bf can lead to PND but I don't think choosing not to bf does."
The research suggests that women who CHOOSE not to breastfeed have significantly higher rates of postnatal depression than women who choose to breastfeed and manage to do it. It's also worth taking into account that it's not personal mores which primarily influence women's feeding intentions in the UK, but their age, education, social class, geographical region and ethnicity. A 17 year old from Yorkshire is likely to formula feed, not because it suits her parenting style, but because that's what all her friends do, and she has never met anyone who's breastfed successfully. A 37 year old Ghanaian solicitor from Hampstead is overwhelmingly likely to breastfeed because she feels it's expected, and it feels 'natural' because all her friends and family have done it.
"As for the others, there are risk factors that matter more."
Oh for goodness sake, do you apply this reasoning to other choices you make for your children? Do you say, 'no don't worry about wearing that bike helmet, as you're more at risk of fracturing your skull from falling off a wall than cycling to school?' Is that how you work out how to keep your children as safe as possible? That reducing one risk somehow 'neutralises' the impact of exposing them to a different one?
"The benefits are pretty marginal. And a 50% risk reduction doesn't mean much unless you post the risk. A reduction of 10% to 5% significant. A reduction from 0.25% to 0.12% not so much"
It's a reduction in risk, using something which is free, and which is part of the normal physiology of reproduction that almost everyone can do. Oh, and the lifetime risk of breast cancer is 1 in 8 in the UK, so a reduction of risk of 50% is far from being 'marginal' wouldn't you say?