It comes down, as do so many discussions about education, to whether you think education can actually make a real, important difference to someone (or whether, on the other hand, you think ability is for practical purposes innate, and will out).
If you don't think education can make a real important difference, then when you nevertheless see one school getting better results than another, and you can't explain it by saying "ah, they were selective at intake" you naturally assume it is hot-housing/spoon-feeding/disparaging description of choice - that is, that they are getting results, but they still haven't made a real important difference to the children getting those results, because you think that's impossible. If you think this, you are also well set-up to disparage people who care what school their children go to, because after all, bright children will do well wherever they go (and presumably, though this part is seldom said, un-bright children will do badly wherever they go)... You are also well positioned to criticise universities for having an intake that includes disproportionate numbers of children from one kind of school, as you believe it to be impossible that children from that kind of school are, by virtue of their education, better fitted for university in a way that actually matters long term.
If you do think education can make a difference, it follows that which school your child goes to may be important, and that it may be the case that a school that gets better results is making a more positive difference. Life is complicated by the fact that it is still possible, in an individual case, that apparently good results are being obtained by hot-housing/spoon-feeding/disparaging description of choice; that just has to be investigated.
Settling the matter of whether education can make a real, important difference or not is not that easy. Personally I'm convinced that it can. To me, therefore, it's very important that my child gets as good an education as I can possibly arrange (which indeed, in my case is an excellent prep school, though in different circumstances it might have been a state school). Although my wording above already made it clear what I think, I think the other point of view is defensible: it's very difficult to examine the contribution of a school, distinguished from all the other influences on the children, and I don't believe it's been done really convincingly in any research I've seen so far, so it comes down to individual intuition. It does always take me aback when I find someone who works in education and seems to believe that education can't make a real, important difference, though - this belief makes their career choice surprising to me!