Evildead, the fact is that a shaven muff DOES look prepubescent! Why can't you admit that?
As for the youth thing, aren't these young women with bare muffs also plastering anti-wrinkle creams on their dewy faces? I argue that in porn it's about youth and this has filtered down into mainstream society.
Your question about sexual pleasure is a good one; I would say that that in itself doesn't negate its value. But, as someone else said here, I don't find that enough of a reason to shave it all off.
There is a footbinding aspect to this issue of excessive over-preening: the hair, brows, eyes, teeth, chin, pits, skin, muff, nails, clit, arse, back, hands, feet - the list goes on. It results in an awful lot of time and attention being focused on maintaining an impossibly high-maintenance body, potentially to the point of narcissism. Time and attention which could be spent much more fruitfully on pursuits which would have a far more constructive and beneficial impact than just making someone look pretty.
Now don't read extremes into what I've just said. I think it's important to look after oneself, and too feel good about oneself. But we have to keep this, like everything, in perspective. Our culture is constantly urging us towards narcissistic self-obsession to get us to spend more money on stuff we don't need. That is a sickness.