<An f/2.8 lens on DX will act like a f/4 lens on FX, because, in order to achieve the same framing and composition, one will either need to shoot DX at a wider angle, or get farther away from the subject. >
This is the main reason I see FX as being preferred for wildlife if you have enough reach. The subject isolation of shooting a longer focal length for the same framing is superior if you are looking for a smooth, blurred background.
Now the assumption is you have to have enough reach to fill the frame. If you lack reach and are simply cropping the image, you lose this benefit. Historically, the issue then shifts to pixels on the subject and other factors such as ISO.
We have had several threads talking about the relative benefit of the large image of the D800 reducing noise. That benefit is reduced and eliminated as you move to smaller crops - and at a DX crop mode is essentially the same as the D7000. But the relative comparison with cropping a D3 or D700 will be unchanged since those FX bodies require a crop as well.