J.R., it could very well be that there is an AFMA or focus shift phenomenon going on with your 50mm on the 6D. It just sounds to me like you're complaining about not getting sharp focus, rather than the side points not achieving focus lock. I had thought you meant those points refused to ever focus at all in your low light situation. And are these children on the sofa, part of a paying gig, or are they just casual shots of your own kids? If they're just casual shots, then is it really such a big issue?
I mean, let's face it, there are types of focus shift even when you focus a fast lens manually, let alone with autofocus.
As for complaining that Canon "hobbled the 6D"...if they did, it was not for fear of impacting sales of the 5D3. It was to ensure that the Nikon D600 could never be offered at a lower street price than the 6D (because Canon's frontend cost on the 6D is lower than the Nikon's for the D600). I thought this was common knowledge? The 6D's feature set has nothing at all to do with the 5D3...think about it.
It just seems like your point in this thread, is to complain about the 6D. You admit you bought it as your backup camera. Perhaps you should sell it, and let someone have it who likes it more...and just buy another used or new 5D3? With the amount of time you've spent complaining about the 6D in this thread, you could have gotten some more photo jobs to pay for the extra 5D3, since you love it so much. No offense of course, I'm here to help :-D ...
It seems to me, that the value of used 5D3's has gone up, since ML have hacked it for RAW video. So that's a good thing for 5D3 owners.
I frankly detest the 5D3, and I currently have no need to do serious video. The next Canon body I buy, will likely be the replacement for the 1DX. Because the 1DX is almost right...
I use FoCal for AFMA so that is not really the issue. There is no focus shift because it should show up on the 5D3 as well - only that it doesn't. The sensitivity of the outer focus points is insufficient indoors and that is the point I was making. This basically results in using the center point only in low light and hence the darned focus - recompose - shoot.
Not getting sharp shots is an issue even if they are casual shots. Do you not want the best IQ if you are taking pics of your kids?
I might appear to be complaining about the 6D but that's really not the case. The thread is on the issue of the AF of the 6D and not the IQ of the 6D. I'm just saying the way it is. Maybe you don't want better AF but not everyone has the same expectations / usage.
I wish I was getting paid for my photos. I do this as a hobby and have no time to turn it into a career. I do agree with your advice and intend to sell the 6D sometime next year and get a 1DX. Thanks!