Hmm, I might be missing some things due to being a newbie...
1) If the ambient exposure is say 1/400s f/1.8 at minimum ISO, how does one use flash when the sync speed is 1/250s without a) closing aperture, b) using HSS or c) using NDs? When you say you -1 the ambient without using HSS, how do you achieve this if your ISO won't go any lower, you've hit the sync limit, and you still want a wide aperture? NDs?
2) Why do folks like Joe McNally end up using so many speedlites to get enough HSS power if a single one is capable of being the dominant contributor to exposure instead of the sun? His complaint was specifically lack of power, not recycle time or battery life. Sure we can concentrate the little flash source even more, but I thought most of the time the idea is to diffuse the main light thus reducing its light intensity?
Do you really run into situations where at say, ISO 100, you're at 1/400 at f1.8? That's very bright light for ISO 100.
Do you specialize in photographing people in front of space-shuttle engine launches or nuclear explosions? ;-)
If you go to the bother of running HSS, it's pretty pointless to then diffuse the flash. If you're going to go that far, it might just be more reliable, and more controllable to have a Lastolite or similar reflector pointed at your subject.
Keep in mind though that those Better Beamer extenders can be bought in a hybrid fresnel lens that actually increases the beam-width substantially. It's incredibly uncomfortable to be photographed with one (your subject sees a giant column of light headed at them - they will see stars/spots) but it works.
Joe McNally is a Nikon CLS master (aka Nikon Ambassador) and is provided with entire iTTL CLS kits for any assignment he gets. He isn't always using HSS when he loads up with iTTL flash units. He does LOVE point source lighting and creates inspiring photos with it. But I have lots of his older work that really predates HSS and CLS/iTTL, and it's all equally impressive.
He chooses to use Nikon CLS flashes instead of monolights for everything he can. HSS doesn't really factor into how or why he uses the stuff, he just flips on the setting (and in some cases the Nikon's will automatically go into HSS mode above sync speed) because he wants it. Also, the flash power you give up is a bit overrated. If you're shooting at 1/2000 with FP HSS, yes, you actually do lose quite a bit of power. It's still useful, but you'll notice. Going from 1/250 to 1/400 though, you won't really notice THAT much of a difference in power available.
At any rate, if you want to darken your background independently of the flash, that's just ambient metering exposure compensation. Some cameras combine flash and ambient compensation though - Olympus has a specific menu item in one of the spanner menus to de-couple the two... Panasonic should work the same. For years, Nikon didn't, yet Canon did. So on Nikon, you had to +1 your flash if you intended to -1 your ambient, and so on. They changed that recently (I think with the D300).