Coming from two Canon 5d MK II bodies and killing one, which I replaced with a mk III. I'm seriously considering selling my remaining mk II and getting a 6D as a second (walkaround and backup) body. The low light AF (dis)abilities of the mk II really bothered me since its release and reports of the 6D are very positive in this regard. I also have the cash for a second MK III but at this point I find it a little overkill to use a 3K camera as a walkaround body dangling over my shoulder or wrist wherever I go, but the 5D Mk III is a JOY to use and love the images I take with it. It is a fantastic camera and the images looks more "natural" to my eyes than the the 5d mk II ever did. And reports that the IQ of the 6D is at least as good as the 5d mk III gives me a reason to ditch the 5d II for one of these two bodies. I can't choose!!
That's probably the best case I've heard so far for a 6D. If it's really just a backup and for walkaround I see how a smaller lighter plastic camera makes sense and its limitations probably won't be too difficult to deal with since you still have the Mark III for the more serious stuff.
On the other hand I personally would keep the second 5DII until fails before spending more money. I still think that the MarkII beats the 6D overall - but then I was never concerned with the AF performance and find it plenty good even in difficult light.
How about something completely different for everyday use/walkaround? I'd consider a Fuji or Sony and keep the 5DII around as second SLR body.
The 5D MII is not better. 1/180 & 1/4000 max shutter is not much of a difference. The 6D is the better camera, especially in low light photography (concerts, clubs, night shots etc) it has 2 full stops of ISO advantage, plus it's a cleaner image. Faster FPS, less shutter lag, better screen, 2 more AF points and better AF in general. If you would list out the differences the advantages are clearly to the 6D. Also especially if you're in colder the states, like Minnesota, the Wifi is so useful. Frame your shot and go back into your car while taking pictures, that comes INCLUDED.