Of course if we want the maximum amount of detail and lower noise we must fill the frame, but that is not always possible as we either have a prime lens on or our zoom is not long enough, so we crop. Modern sensors, such as my 7D's 18MP, can handle a lot of cropping (at lower ISOs), so why whould I not?
On the otherhand cropping is commonplace for film and video, especially as there are differing aspect ratios, a wide 19:6 capture must be cropped down for a 4:3 display.
Eventually it comes down to the photographer to decide on what he wants to show. Like here, I could have cropped down to the bunny and still had enough resolution for an A4 print, but I wanted to show some of his environment, set the scene, leave space for a message incase someone decided to buy it as an easter card ;)
50D, 100-400, ISO200
It is all down to the artists vision, let them decide what should be seen.