"why all the silly headroom?"
Because I want big, clean, dynamic power.
If you want some of that "grit/grind/extra harmonics" or whatever you choose to label it that power sections make when they get pushed, that's fine.
I don't.
I mostly don't have to play Really Loud; but, when I need to, I want it to keep clean and still have the ability to be dynamic and responsive.
I have cabs capable of dealing with the 800 watts of my highest rated head, as long as I don't abuse that.
Honestly, you can "blow up a 1500 watt rated cab with a 500 watt rated head" if "you're doing it wrong," but, pretty much this whole post isn't what this thread started out about, so, I'll quit now.