Can anybody submit a nice picture that did require this -3EV capability?
In response to this:
The autofocus determines the correct focus through contrast of edges, and when there's not enough light transmitted through the lens (TTL) the autofocus can't lock. Most cameras have the ability to autofocus above 0EV up to around +20EV, so in very bright conditions.
But focusing in the other direction determines at which point the camera fails to obtain a lock with a negative EV, or in lower light.
Most cameras are around 0EV (a7R), -1EV (D600), and -2EV (5D3 & D800E). The lower the EV the more accurate the camera can obtain a lock in low light. Low light could mean it's dark out, or you have an ND filter and it's daytime.
The 6Ds -3EV doesn't have much real world applicability in my opinion outside of using ND filters. For example right now I have a 6-stop ND filter stuck on the end of my GND adapter ring. So I'm constantly working with a dark viewfinder (until I get the thing unstuck or buy another adapter ring :| ) and the -3EV does have a noticeable effect - 1 stop greater for autofocus than my D800E or 5D3.
So it's an incredibly small thing that has a narrow benefit for some photographers, but it's not that big of a deal in and of itself (in my opinion).