Since the 6D is post-600EX-RT, I assume there would be some advantage to having a 6D over a 5DII for HSS off-camera, though I know some people are not feeling limited by the 5DII in that regard. The only other possible advantage I can think of is that if the sensor + digic V more closely matches the color and noise of the 5DIII, it could be a capable back-up camera for me...but I'll still buy another 5DIII.
If I was a hobbyist primarily into portraits, I'd be thinking the 6D+85L combination looks pretty good at $4,100 versus $3,900 for a 5DIII + 85 f1.8. For the photographer that wants to go FF, but have money left over for glass, I imagine the 6D will be a nice option once the 5DII is nola. For that matter, you could buy two expensive 600EX-RT's to go with a 6D and still come out less than a 5DIII....not ahead, just less.