Why aren't you using it on your D600?
Because no one makes a zoom that goes to 900mm. Because I can afford both a D600 and a D7100, but understand that not everyone can as well. Because maybe there are those who want/use/need 150-600mm on an FX body.
Blanket statements like this can cause more harm than good. I certainly wouldn't put this lens on a D40 for shooting sports or wildlife.
Now, please answer my question.