The200-400 is not even a birding lens, that is what the 600mm and 800mm lenses are for. The 200-400 is a great large wildlife lens for when you never know what you will get or how close you might eb. As such it is the ultimate safari lens but is very popular for large game animals in places like Yellowstone when sometimes you do get close to bears etc.
I am actually not a big fan of the 200-400mm lens. 200mm f/4.0 is very slow and disappointing if you have a 70-200mm f/2.8, 300mm f/4.0 is also nothing special and quite affordable (you can buy older models for a 300-500 quid). So you are really paying the money to have 400mm @ f/4.0 and the ability to pull back. 400mm f/4.0 is a nice step up form the affordable 300mm f/4.0 but you don't get enough increase in length to justify the cost IMO.
Either never felt the need to pull back really but I know at time it can certainly be very helpful, e.g. on Safari and stuck inside the landrover.
shooting a 300m f/4.0 with 1.4xTC on a 1.5x DX crop (630mm equivalent) I rarely felt I had too much reach but almost always wished for more. Taking a few steps back from the large friendlier birds is sufficient, but I rarely had to do that.
I can really see why people pay for the 800mm lenses if they are really into birds. 630mm equivalent was mostly good enough but required heavy crops if you weren't very lucky. Now I have moved to FF the lack of reach is really apparent. Luckily the pixel density of the D800 is sufficiently high such that a heavy crop still yields plenty of detail.