You must shoot tiny birds, which is the only real advantage of crop, IMHO. I shoot grizzlies, bighorn, moose, elk, and raptors in the wild (not zoos, game farms, or fenced in areas). The differences in IQ, smooth exposures, and general harshness and roughness to the overall image is enormous.
I'm in the field now, just outside the largest wilderness complex in the lower 48, waiting for Canon to send my broken FF back to me. It should be here tomorrow.
Smaller creatures yes - live in Victoria Australia, so smaller birds, lizards, small marsupials etc tend to be the focus, and I shoot underwater too.
If I can get close enough, the 6D is pretty good. But once AF or distance enters into it, 7D works better for me, let alone the 2. I do sometimes wonder if a second hand 5DIII might have been the better choice, but once I start looking at the extra cost lenswise to cover what I currently have, not so much. Crop vs FF isnt just about the body cost, by a long shot.