Have you ever shot under crappy stadium lighting? Two words: flicker reduction.
That alone was worth the price of upgrading from the 7D to the 7D Mark II. Going from having about 7 out of every 10 frames ruined by the differences in light from one side of the frame to the other in terms of brightness and color, seeing color temp vary from one side of the frame to the other, seeing it vary by as much as 500-800K (also on the M <---> G axis) from one frame to the next, getting really ugly narrow spectrum light at the darkest point, and having the overall exposure vary by as much as two stops from peak to trough in the lighting cycle, to having every single frame look the same in terms of brightness and color is transformative.
Where shooting raw and custom post-processing of each individual frame was needed to get color and exposure remotely right, now the results are consistent enough to do batch processing or even shoot straight to JPEG and be able to use a much higher percentage of the frames taken.
Not only that, but since the camera times the shutter release with the peak of each light cycle I can now shoot at 2/3-1 stop faster under the same lights! Where I used to shoot at 1/500, f/2.8, ISO 2500 I can now shoot at 1/1000, f/2.8, ISO 2500, or 1/640, f/2.8, ISO 1600, etc. This also increases the keeper rate.
The much improved AF of the 7D Mark II compared to the 7D made it usable, but I'd love to have this feature in a FF body without springing $6K for the soon to be released 1D X Mark II. As improved as the AF in the 7D Mark II is over the 7D, it's still not as consistent from shot-to-shot as the 5D Mark III. The narrower baseline of the APS-C sized mirror and PDAF array limits it in terms of accuracy compared to its FF counterparts.
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Flicker reduction. That's what I was thinking. I can use a low light body and prefer full frame. The 5 series being more affordable is a Canon bread and butter body. I trust they'll get it right and buyers will be quite satisfied.