Is crop mode that important in video? (I just dont know enought about video to know)?
In the universal case, for nature photography every little bit helps like mad, unlike with stills, there are not enough MP to be able to crop down later on and zoom in on anything.
In the universal, but not for everyone case, it can be useful for those who want to use cinema lenses made for non-vista vision-type movie cameras. The avg use might not use that too much due to $$ of many of those lenses though.
And in the specific 5D3 case some find that it is not quite as sharp and detailed as expected, some speculate perhaps the large 3x3 sampling means heavy smoothing to avoid aliasing and a 2x2 C300-like sampled mode closer to the native AA filter strength might mean not much smoothing need be applied and perhaps a crisper, more detailed final results (a the trade off of over a stop of SNR lost and no FF DOF wide open, so it would certainly need to be an additional mode, not the only mode).
If the 5D3 is detailed enough and you are shooting all set scenes that fit your available focal lengths with ease and sticking to regular FF SLR lenses then it doesn't matter at all.