> Can you explain why you think there is sufficient additional value in completely---absurdly so---free speech (as per P's scenarios, for example) over limiting obviously dangerous and detrimental speech?

Not simply, no.

It's a very long (book sized) argument that I've been gradually making bits and pieces of over the years. I really wish I'd kept the little bits I'd type out in one forum or another so I could just point at that...

Though, for a fragment and possible intuition pump: the censoring is putting the cart before the horse. You need to determine whether something is dangerous or detrimental before banning it, but the act of censoring it or neighbouring ideas or the kind of people who espouse it prevents that very determination that ensures you aren't censoring true things. You also get all sorts of other effects like purity spirals and fights over which group gets to decide what is censored and groups then doing things even they admit is wrong in order to keep control of the censors ("it's worth a little lie to stop the nazis taking control").

EDIT: You're correct that the comment about kids isn't a full argument, it's an argument fragment that needs various other things to be held and explained in order to be complete. But there's no point in spending ages outlining it all unless I'm going to do so in a less transient medium like a blog post. So instead I post the fragment hoping it's the one piece the reader was missing for things to click.