As I've said many times, due to the low noise of the 6D, the low light sensitivity of the center AF point, can be very useful in the majority of situations where you are shooting wildlife (or people) around, before, or after sunset. Or else if you are shooting landscape hand-held, with an IS lens, up to an hour after sunset...or during a full moon. Or if you are shooting inside a club, or outside on a dimly lit city street at night, that -3EV capability is very useful.
ISO 6400 is extremely usable for professional prints via the 6D (with a bit of post processing), and ISO's a bit above that are still useful.
ISO 6400 at -3 EV would be 1/4 s at f/1.4, 1 s at f/2.8. Are you actually suggesting that such settings are, "useful in the majority of situations where you are shooting wildlife (or people) around, before, or after sunset
??" In that case, I would expect you'll have plenty of examples of wildlife that you shot at 1 s or longer exposures (unless you shoot a lot of wildlife with f/1.4 lenses) and people that you've shot at exposures of 1/4 s or longer, and you should be able to share several of them to support your contention.
Assuming 1/FL, a 24-28mm lens with 4-stop IS could be handheld at ~1/4 s (a conservative assumption, as with today's sensors 1/2xFL is more realistic). That means your handheld landscape at -3 EV ISO 6400 would be at f/2.8 (and there are a few wide angle f/2.8 lenses with 4-stop IS/VC). I'd be interested in seeing some examples of your landscapes shot at those settings, too, assuming you have any.
I've personally never shot landscapes/cityscapes at -3 EV unless on a tripod. The shot below with the swan at the Palace of Fine Arts in San Francisco is the closest I've come to that handheld, it's 1/25 s, f/3.2 and ISO 12800, which is -1.3 EV. There are issues with the shot - the DoF is too shallow so the structure is OOF, and the shutter speed wasn't quite fast enough to freeze the motion of the swimming swan; IS would not have helped (and at 17mm on a FF camera, IS isn't even a possibility). The second shot below is an example of -3 EV - the only light sources in the room were the fireplace and the jack-o-lanterns, the settings were 1/15 s, f/2.8, ISO 102400. In fact, I focused that shot through the viewfinder (although I could barely see), meaning the 1D X's AF was actually able to focus at -3 EV (although that's for the whole scene, the AF point was seeing more light than that). Still, there's motion blur and lots of noise, and image is no more than a snapshot (albeit one not possible with most cameras).
Considering the examples above, I would suggest that anyone arguing the -3 EV sensitivity of the 6D's AF system is an advantage over the -2 EV of the 5DIII or 1D X, or even the -0.5 EV sensitivity of most other bodies, in many situations, much less a majority of situations, doesn't know what they're talking about.
As I stated, the majority of situations which people call "low light" really offer plenty of light for the AF systems of even low-end dSLRs. The extra one stop of sensitivity that the 6D has is an advantage only in very rare situations, so rare that many 6D users probably have not ever experienced them.
As for bashing the other AF points on the 6D, you need to bash the 5D2's as well, because they were no better. It might not still be on sale, but plenty of forum readers still own and use the 5D2.
I agree, the outer AF points of the 5DII were not very useful, much like those of the 6D. They did ok with high contrast subjects in bright light, but were lousy in dimmer light or for Servo tracking.