I am using 70-200mm and 300mm f4L lens. I think the main issue is not with the camera body but with the lenses. When you use a fx lens on a dx camera, you get very similar result... although I do recognised that most FF camera produce more details as compared to cropped sensor with similar mega-pixels... and I also recognised that most cropped sensor do not produce as clean an image as compared to a FF sensor of the same era.
I think most people think that cropped sensor is still desirable for wildlife not just because of the crop factor... but precisely of the deeper DOF, which can be easier to get the desirable result. Of course there is nothing wrong with using the FF for wildlife shots. Like you have also mentioned, in the hands of skilled photographer, using crop sensor camera can also produce great images, it just take more effort and in my opinion, it is that extra effort that can make things fun. After all, alot of people are not professionals who wanted to earn a living out of photography.
oh... and I do a bit of surfing...
This guy is using a 1100D entry level DSLR with a entry level zoom lens (55-250mm) and the image is spectacular too.