the 3x crop mode is decidedly soft compared to the standard video; it has less artifacts (no line-skipping-induced aliasing/moire), but if you see tests of 3x crop vs zooming with the lens, the 3x crop is comparatively soft:
that's because, even if it is 2.7x instead of 3x, as pgabor says, it's too much: you need at least a 2.5K bayer pattern to create a sharp 2K image
still, if you need it, it's really useful; but it has limitations
Isn't the digi-crop (not the 'zoom' that goes beyond 2.7x or 3x or whatever the true number is) the same as cropping a still the same amount in post?
The first video example is thru a 100 L and it is in a different league than the second due to the lens, just like a 3x crop of a still shot thru a 100 L or some other hi-resolving lens is in a different league than any "short to short-tele" zoom.
Now I would agree that 3x on APS-C has it's limitations for the reasons stated, but using a stellar lens will make all the difference and it becomes quite useable as seen in the first video. And that is the same as I see when cropping a 100 L 3x vs cropping a 24-104 L at 100mm the same way (which is a better zoom than used in the second video).
So I'd have to agree that I'd love to see it on a 5d3 because that body hopefully will have enough more pixels to make the 3x fantastic when cropped in similar proportion. It would greatly expand the toolbox on the video side.