I don't see it suggested here, but the 1.3x crop is a bonus vs. the 1Dx or 5DIII(for sports/wildlife).
That's the main reason I have my 1DIV. My 300 pretty much lives on it. If only had the 5DIII or 1DX I'd need a 400 which is just to big and heavy.
I pretty much only use the 1DIV for things that need speed and/or reach, often I'll shoot 2 bodies with a zoom on the 5DIII, and the 5DIII gets used for pretty much everything else.