You're on about fixed 3 point harnesses, not inertia-type belts like in production cars (doesn't really apply to our types of cars) right?
I get the focus on racing/etc but it's not something the average joe on the road really needs to worry about. In our types of cars, you have to be pragmatic for the general types of users we have; I guess 95% of sales are to road users. An inertia reel isn't practical because of the car design (you could get the mount up on the rollover hoop, sure, if GBS were that way inclined) but I'd probably err more on the side of strapping myself in so I don't come out. Submarining or not, I think if an impact was that serious in our little tin cans you'll be waving goodbye to your legs pretty quickly, whether it's a 3, 4 or 20 point harness :). A frontal is probably not too bad because there's a heck of a lot of front crumple (ie. the entire chassis forward of the bulkhead) but anything remotely side-on, or something that splits a chassis tube and had it poking into the passenger compartment.... I think the least of your worries is "damn it, I wish I had a crotch strap to stop me sinking forward"
Just wouldn't want people worrying over nothing, or at least putting it into context. The guys who popped their testiles out thanks to a 6 point are probably very annoyed about it, sure, but I bet they're glad they were wearing something more than a 3-point intertia reel !!
edit: I should say, you're totally right, a badly-adjusted harness is going to do next to nothing, and at least that's the one good thing about a 3 point, it's less prone to incorrect adjustment. I have an issue with the GBS/TRA lap straps, they constantly loosen, I was speaking to TRS at Autosports about swapping my harness for ones with reversed lap straps with motorsport adjusters on them... bugs the hell out of me