In retrospect, I'm not sure I agree...but that may be particular to my situation. I went out not knowing what settings I'd need for the much-darker-than-normal eclipsed moon. I had my 1D X and my M2 with the EF adapter, and the 600/4 II with the 2xIII. With that narrow an FoV, anything over 0.5 s resulted in motion blur from the earth's rotation, and at the peak of the eclipse I needed ISO 6400 to get that shutter speed. I would not have wanted to be using an APS-C sensor at that ISO.
I wouldn't have been happy with the 5D II, 5D III, 5Ds R or the 7D II at ISO 6400. That is why I went with the 700mm at F/5.6 rather than 1000mm at F/8 to gain a stop.
This eclipse at total was 2 stops darker than the last and the sky was not as clear. With the sky like this it didn't matter to me which body, but if the sky had been clear I would still prefer the 7D II and 5Ds R over the other bodies. Using the 7D II in this situation you lose the ability to go to F/8 and it turns into a version of the old debate, which is better the 2x extender and FF 1Dx or the pixel density and a 1.4x (notice I didn't say crop since you can get the density with a FF now).
With a wider FoV you can also use a longer exposure. But...if you have to crop the image, you increase the apparent noise anyway.